Reland "Enable unused resource removal for android webview"

This reverts commit b4682a7.

Reason for revert: I don't think this was the cause of the tree closure. The tree reopened automatically just before my revert landed.

Original change's description:
> Revert "Enable unused resource removal for android webview"
>
> This reverts commit 0deb4b3.
>
> Reason for revert: Looks like it closed the tree
>
> https://ci.chromium.org/ui/p/chromium/builders/ci/chromeos-arm-generic-dbg/49672/blamelist
>
> The error is about resources.
>
> [1549/76611] ACTION //chromeos/components/hps:hps_internals_ts(//build/toolchain/cros:target)
> FAILED: gen/chromeos/components/hps/tsconfig.json gen/chromeos/components/hps/tsconfig.manifest gen/chromeos/components/hps/resources/hps_internals.js 
> python3 ../../tools/typescript/ts_library.py --root_dir ../../chromeos/components/hps --gen_dir gen/chromeos/components/hps --out_dir gen/chromeos/components/hps --in_files resources/hps_internals.ts --definitions ../../../../../../tools/typescript/definitions/chrome_send.d.ts --path_mappings chrome://resources/\*\|../../../ui/webui/resources/preprocessed/\* //resources/\*\|../../../ui/webui/resources/preprocessed/\* chrome://resources/polymer/v3_0/\*\|../../../../../../third_party/polymer/v3_0/components-chromium/\* //resources/polymer/v3_0/\*\|../../../../../../third_party/polymer/v3_0/components-chromium/\* chrome://resources/polymer/v3_0/polymer/polymer_bundled.min.js\|../../../../../../third_party/polymer/v3_0/components-chromium/polymer/polymer.d.ts //resources/polymer/v3_0/polymer/polymer_bundled.min.js\|../../../../../../third_party/polymer/v3_0/components-chromium/polymer/polymer.d.ts /tools/typescript/definitions/\*\|../../../../../../tools/typescript/definitions/\*
> Traceback (most recent call last):
>   File "../../tools/typescript/ts_library.py", line 153, in <module>
>     main(sys.argv[1:])
>   File "../../tools/typescript/ts_library.py", line 117, in main
>     node.RunNode([
>   File "/b/s/w/ir/cache/builder/src/out/Debug/../../third_party/node/node.py", line 34, in RunNode
>     raise RuntimeError('Command \'%s\' failed\n%s' % (' '.join(cmd), err))
> RuntimeError: Command '/b/s/w/ir/cache/builder/src/out/Debug/../../third_party/node/linux/node-linux-x64/bin/node /b/s/w/ir/cache/builder/src/out/Debug/../../third_party/node/node_modules/typescript/bin/tsc --project gen/chromeos/components/hps/tsconfig.json' failed
> ../../chromeos/components/hps/resources/hps_internals.ts:6:32 - error TS2792: Cannot find module 'chrome://resources/js/cr.m.js'. Did you mean to set the 'moduleResolution' option to 'node', or to add aliases to the 'paths' option?
>
> 6 import {addWebUIListener} from 'chrome://resources/js/cr.m.js';
>                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> ../../chromeos/components/hps/resources/hps_internals.ts:7:17 - error TS2792: Cannot find module 'chrome://resources/js/util.m.js'. Did you mean to set the 'moduleResolution' option to 'node', or to add aliases to the 'paths' option?
>
> 7 import {$} from 'chrome://resources/js/util.m.js';
>
>
> Original change's description:
> > Enable unused resource removal for android webview
> >
> > Bug: 1269757
> > Change-Id: Ib3077a1c56f32b8de5df391ee19aae1790ae507f
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3534059
> > Auto-Submit: Mohamed Heikal <mheikal@chromium.org>
> > Reviewed-by: Richard Coles <torne@chromium.org>
> > Reviewed-by: Andrew Grieve <agrieve@chromium.org>
> > Commit-Queue: Andrew Grieve <agrieve@chromium.org>
> > Cr-Commit-Position: refs/heads/main@{#984640}
>
> Bug: 1269757
> Change-Id: I8811142db0fb070881aafa68e7ecc2d0ef594635
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3546167
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Owners-Override: Fergal Daly <fergal@google.com>
> Commit-Queue: Fergal Daly <fergal@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#984657}

Bug: 1269757
Change-Id: I4cdd3dc853c466876faee12d5d1572590dd5f3c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3546139
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Fergal Daly <fergal@google.com>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#984677}

android_webview/system_webview_apk_tmpl.gni

@@ -231,6 +231,13 @@ template("system_webview_apk_or_module_tmpl") {
 
       # Shortens resource file names in apk eg: res/drawable/button.xml -> res/a.xml
       short_resource_paths = !is_java_debug
+
+      # Removes unused resources from the apk. Only enabled on official builds
+      # since it adds a slow step and serializes the build graph causing fewer
+      # expensive tasks (eg: proguarding, resource optimization) to be run in
+      # parallel by adding dependencies between them (adds around 10-20
+      # seconds on my machine).
+      strip_unused_resources = is_official_build
     }
 
     if (!_is_bundle_module) {

android_webview/system_webview_bundle.gni

@@ -88,6 +88,9 @@ template("system_webview_bundle") {
       proguard_android_sdk_dep = webview_framework_dep
     }
 
+    # For this to be respected, it must also be set on the base module target.
+    strip_unused_resources = is_official_build
+
     forward_variables_from(invoker, "*")
   }
 }

build/android/unused_resources/UnusedResources.java

@@ -382,7 +382,10 @@ private void recordClassUsages(File file, String name, byte[] bytes) {
         ReferenceChecker callback = new ReferenceChecker() {
             @Override
             public boolean shouldProcess(String internalName) {
-                return !isResourceClass(internalName + DOT_CLASS);
+                // We do not need to ignore R subclasses since R8 now removes
+                // unused resource id fields in R subclasses thus their
+                // remaining presence means real usage.
+                return true;
             }
 
             @Override
@@ -400,6 +403,12 @@ public void referencedStaticField(String internalName, String fieldName) {
                 Resource resource = getResourceFromCode(internalName, fieldName);
                 if (resource != null) {
                     ResourceUsageModel.markReachable(resource);
+                    if (mDebugPrinter != null) {
+                        mDebugPrinter.println("Marking " + resource
+                                + " reachable: referenced from dex"
+                                + " in " + file + ":" + name + " (static field access "
+                                + internalName + "." + fieldName + ")");
+                    }
                 }
             }