Revert "Remove TabRanker."

This reverts commit 979b218.

Reason for revert: Main suspect for causing a build breakage at https://ci.chromium.org/ui/p/chromium/builders/ci/chromeos-octopus-rel/8968/overview

Original change's description:
> Remove TabRanker.
>
> It is not used.
>
> Bug: 784639
> Change-Id: Idd4dc6f0dd8603d3e51b671aab68d761c90580f1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4064839
> Reviewed-by: Charles Zhao <charleszhao@chromium.org>
> Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
> Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1084960}

Bug: 784639
Change-Id: I735b21c09f9af16a3e6f840d851ae287a2bf9c32
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4116477
Owners-Override: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Iman Saboori <isaboori@google.com>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1085045}

chrome/browser/BUILD.gn

@@ -4399,6 +4399,8 @@ static_library("browser") {
       "//chrome/browser/policy:path_parser",
       "//chrome/browser/profile_resetter:profile_reset_report_proto",
       "//chrome/browser/resource_coordinator:intervention_policy_database_proto",
+      "//chrome/browser/resource_coordinator:tab_metrics_event_proto",
+      "//chrome/browser/resource_coordinator/tab_ranker",
       "//chrome/browser/resources:component_extension_resources",
       "//chrome/browser/share/core:share_targets",
       "//chrome/browser/share/proto:proto",

chrome/browser/ash/BUILD.gn

@@ -3323,6 +3323,7 @@ source_set("ash") {
     "//chrome/browser/image_decoder",
     "//chrome/browser/policy/messaging_layer/proto:crd_event_proto",
     "//chrome/browser/profiles:profile",
+    "//chrome/browser/resource_coordinator:tab_metrics_event_proto",
     "//chrome/browser/safe_browsing",
     "//chrome/browser/ui/webui/ash/crostini_upgrader:mojo_bindings_headers",
     "//chrome/browser/ui/webui/bluetooth_internals:mojo_bindings",

chrome/browser/browser_resources.grd

@@ -325,6 +325,10 @@
         <include name="IDR_RESET_PASSWORD_JS" file="${root_gen_dir}\chrome\browser\resources\reset_password\tsc\reset_password.js" use_base_dir="false" type="BINDATA" />
         <include name="IDR_RESET_PASSWORD_MOJOM_WEBUI_JS" file="${root_gen_dir}\chrome\browser\resources\reset_password\tsc\reset_password.mojom-webui.js" use_base_dir="false" type="BINDATA" />
       </if>
+      <if expr="not is_android">
+        <include name="IDR_TAB_RANKER_EXAMPLE_PREPROCESSOR_CONFIG_PB" file="resource_coordinator\tab_ranker\example_preprocessor_config.pb" type="BINDATA" />
+        <include name="IDR_TAB_RANKER_PAIRWISE_EXAMPLE_PREPROCESSOR_CONFIG_PB" file="resource_coordinator\tab_ranker\pairwise_preprocessor_config.pb" type="BINDATA" />
+      </if>
       <if expr="chromeos_ash">
         <include name="IDR_SMART_DIM_20190521_EXAMPLE_PREPROCESSOR_CONFIG_PB" file="ash\power\ml\smart_dim\20190521_example_preprocessor_config.pb" type="BINDATA" />
       </if>

chrome/browser/chromeos/BUILD.gn

@@ -172,6 +172,7 @@ source_set("chromeos") {
     "//chrome/browser/policy/messaging_layer/proto:crd_event_proto",
     "//chrome/browser/profiles",
     "//chrome/browser/profiles:profile",
+    "//chrome/browser/resource_coordinator:tab_metrics_event_proto",
     "//chrome/browser/resources/chromeos:app_icon_resources",
     "//chrome/browser/resources/chromeos/cloud_upload:resources_grit",
     "//chrome/browser/resources/settings:resources_grit",

chrome/browser/resource_coordinator/BUILD.gn

@@ -6,6 +6,10 @@ import("//chrome/common/features.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//third_party/protobuf/proto_library.gni")
 
+proto_library("tab_metrics_event_proto") {
+  sources = [ "tab_metrics_event.proto" ]
+}
+
 mojom("mojo_bindings") {
   sources = [ "lifecycle_unit_state.mojom" ]
   webui_module_path = "/"

chrome/browser/resource_coordinator/tab_metrics_event.proto

@@ -0,0 +1,82 @@
+// Copyright 2017 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+package metrics;
+
+// Defines specialized fields used by the TabManager.TabMetrics UKM event.
+message TabMetricsEvent {
+  // Represents the MIME type of content.
+  // MIME type strings have the format "<type>/<subtype>". The values for
+  // ContentType reflect just the top-level type (e.g., AUDIO is used for both
+  // "audio/mp3" and "audio/ogg").
+  // The exception is "text/html", since it's treated quite differently than
+  // other text.
+  enum ContentType {
+    // Could not be determined or not available.
+    CONTENT_TYPE_UNKNOWN = 0;
+
+    // Not one of the types below.
+    CONTENT_TYPE_OTHER = 1;
+
+    // The "application/*" type. Supported application sub-types are enumerated
+    // in Chrome's PluginType.
+    CONTENT_TYPE_APPLICATION = 2;
+
+    // The "audio/*" type.
+    CONTENT_TYPE_AUDIO = 3;
+
+    // The "image/*" type.
+    CONTENT_TYPE_IMAGE = 4;
+
+    // The "text/*" type. For "text/html", TEXT_HTML is used instead.
+    CONTENT_TYPE_TEXT = 5;
+
+    // The "text/html" type, representing web pages.
+    CONTENT_TYPE_TEXT_HTML = 6;
+
+    // The "video/*" type.
+    CONTENT_TYPE_VIDEO = 7;
+  }
+
+  optional ContentType content_type = 1;
+}
+
+// Defines specialized fields used by the TabManager.WindowMetrics UKM event
+// for describing the window used by a set of tabs.
+message WindowMetricsEvent {
+  // Represents the type of the window.
+  enum Type {
+    // Could not be determined or not available.
+    TYPE_UNKNOWN = 0;
+
+    // A normal browser window with a tab strip.
+    TYPE_TABBED = 1;
+
+    // A pop-up browser window with no tab strip.
+    TYPE_POPUP = 2;
+
+    // An Extension browser window that is not an AppWindow.
+    TYPE_EXTENSION = 3;
+
+    // An AppWindow for a platform app.
+    TYPE_APP = 4;
+  }
+
+  optional Type type = 1;
+
+  // Represents the show state of the window.
+  enum ShowState {
+    SHOW_STATE_UNKNOWN = 0;
+    SHOW_STATE_NORMAL = 1;
+    SHOW_STATE_MINIMIZED = 2;
+    SHOW_STATE_MAXIMIZED = 3;
+    SHOW_STATE_FULLSCREEN = 4;
+  }
+
+  optional ShowState show_state = 2;
+}

chrome/browser/resource_coordinator/tab_ranker/BUILD.gn

@@ -0,0 +1,46 @@
+# Copyright 2018 The Chromium Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+source_set("tab_ranker") {
+  # Dependent targets should not include native_inference.h directly.
+  public = [
+    "mru_features.h",
+    "tab_features.h",
+    "tab_score_predictor.h",
+    "window_features.h",
+  ]
+
+  sources = [
+    "native_inference.cc",
+    "native_inference.h",
+    "pairwise_inference.cc",
+    "pairwise_inference.h",
+    "tab_features.cc",
+    "tab_score_predictor.cc",
+    "window_features.cc",
+  ]
+
+  public_deps = [ "//base" ]
+
+  deps = [
+    "//chrome/browser:resources",
+    "//chrome/browser/resource_coordinator:tab_manager_features",
+    "//chrome/browser/resource_coordinator:tab_metrics_event_proto",
+    "//components/assist_ranker",
+    "//components/assist_ranker/proto",
+    "//components/sessions",
+    "//services/metrics/public/cpp:ukm_builders",
+  ]
+}
+
+# target for using GetFullTabFeaturesForTesting in unit tests.
+static_library("tab_features_test_helper") {
+  testonly = true
+  sources = [
+    "tab_features_test_helper.cc",
+    "tab_features_test_helper.h",
+  ]
+
+  deps = [ ":tab_ranker" ]
+}

chrome/browser/resource_coordinator/tab_ranker/OWNERS

@@ -0,0 +1,3 @@
+amoylan@chromium.org
+charleszhao@chromium.org
+michaelpg@chromium.org

chrome/browser/resource_coordinator/tab_ranker/README.md

@@ -0,0 +1,56 @@
+# Tab Ranker
+
+The Tab Ranker is an experimental model used to predict tab reactivation. We
+are setting up experiments to use this model as a factor in deciding which tabs
+to discard when the system is low on memory.
+
+## Model
+
+The input to the model is a list of metrics about a background tab, such as
+whether it is pinned, how long it has been in the background, and how many
+events it has received. The output is a floating point value, where a higher
+number suggests the tab is more likely to be reactivated than a lower number.
+The range of possible outputs is dependent on the model; what matters is the
+relative ordering of scores, not the precise value of a score.
+
+The metrics used for inference are the same metrics logged to UKM by
+TabMetricsLogger. Using these metrics, we trained a
+[DNN](https://en.wikipedia.org/wiki/Deep_learning#Deep_neural_networks) model.
+The inferencing code in `native_inference.cc` consists of the model weights
+generated by TensorFlow and basic operations to execute the model over an
+example.
+
+See [native_inference.md] for details on how the tf.native code is cleaned up
+for Chromium.
+
+## Example preprocessing
+
+Examples are provided to the model as an array of floats, with each element
+corresponding to a feature. For scalar metrics like WasRecentlyActive(), this
+feature is simply a 0 or 1.
+
+A metric with multiple possible values, like an enum or bucketized number, is
+broken into a number of features, each corresponding to one of its possible
+values. For example, a value of 7 for the PageTransitionCoreType metric is
+represented as a 1 in the `PageTransitionCoreType_7` feature. See
+https://en.wikipedia.org/wiki/One-hot.
+
+The AssistRanker handles this example preprocessing based on the configuration
+provided in example_preprocessor_config.pb. This can be examined with the
+`print_example_preprocessor_config.py` utility:
+
+```
+  ./components/assist_ranker/print_example_preprocessor_config.py \
+  out/Release \
+  chrome/browser/resource_coordinator/tab_ranker/example_preprocessor_config.pb
+```
+
+## Scoring a tab
+
+Use `TabActivityWatcher::CalculateReactivationScore()` to get a score from a
+WebContents.
+
+Internally, `TabScorePredictor::ScoreTab` is the interface to the model. The
+caller needs to track a number of metrics about the tab and its window. These
+metrics must be calculated in the same way they were logged, as otherwise the
+results will not be valid.

chrome/browser/resource_coordinator/tab_ranker/mru_features.h

@@ -0,0 +1,25 @@
+// Copyright 2018 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_RESOURCE_COORDINATOR_TAB_RANKER_MRU_FEATURES_H_
+#define CHROME_BROWSER_RESOURCE_COORDINATOR_TAB_RANKER_MRU_FEATURES_H_
+
+#include <stdint.h>
+
+namespace tab_ranker {
+
+// Identifies how recently the tab was used.
+struct MRUFeatures {
+  // Zero-based, so this indicates how many of the |total| tabs are more
+  // recently used than this tab.
+  int index = 0;
+
+  // Total number of tabs considered when calculating MRU index, ie number of
+  // non-incognito tabs open.
+  int total = 0;
+};
+
+}  // namespace tab_ranker
+
+#endif  // CHROME_BROWSER_RESOURCE_COORDINATOR_TAB_RANKER_MRU_FEATURES_H_