Set receive_header_start only for main frame document

This is to fix a bug in a previous CL.
https://chromium-review.googlesource.com/c/chromium/src/+/4476487. The
receive_header_start_ of the PageLoadTracker object is set for each
resource. We want the field to hold the value of the main frame document
only.

(cherry picked from commit b682b61)

Bug: 1431906
Change-Id: Ifc76f40c1790f626b9aacbece785010c4d6e0710
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4562797
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Hao Liu <haoliuk@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1150323}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4575844
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/branch-heads/5790@{#188}
Cr-Branched-From: 1d71a33-refs/heads/main@{#1148114}

components/page_load_metrics/browser/page_load_tracker.cc

@@ -691,8 +691,15 @@ void PageLoadTracker::FlushMetricsOnAppEnterBackground() {
 
 void PageLoadTracker::OnLoadedResource(
     const ExtraRequestCompleteInfo& extra_request_complete_info) {
-  receive_headers_start_ =
-      extra_request_complete_info.load_timing_info->receive_headers_start;
+  // The main_frame_receive_headers_start_ should be only set once during a
+  // page load. A new page load would have a new PageLoadTracker object.
+  if (extra_request_complete_info.request_destination ==
+          network::mojom::RequestDestination::kDocument &&
+      !main_frame_receive_headers_start_.has_value()) {
+    main_frame_receive_headers_start_ =
+        extra_request_complete_info.load_timing_info->receive_headers_start;
+  }
+
   for (const auto& observer : observers_) {
     observer->OnLoadedResource(extra_request_complete_info);
   }
@@ -938,10 +945,11 @@ void PageLoadTracker::OnTimingChanged() {
                .value());
 
   if (largest_contentful_image_changed) {
-    if (receive_headers_start_.has_value() && !GetNavigationStart().is_null()) {
+    if (main_frame_receive_headers_start_.has_value() &&
+        !GetNavigationStart().is_null()) {
       RecordLargestContentfulPaintImageLoadTiming(
           *paint_timing->largest_contentful_paint,
-          receive_headers_start_.value() - GetNavigationStart());
+          main_frame_receive_headers_start_.value() - GetNavigationStart());
     }
   }
 

components/page_load_metrics/browser/page_load_tracker.h

@@ -572,7 +572,7 @@ class PageLoadTracker : public PageLoadMetricsUpdateDispatcher::Client,
 
   uint32_t soft_navigation_count_ = 0;
 
-  absl::optional<base::TimeTicks> receive_headers_start_;
+  absl::optional<base::TimeTicks> main_frame_receive_headers_start_;
 
   const internal::PageLoadTrackerPageType page_type_;
 

components/page_load_metrics/browser/page_load_tracker_unittest.cc

@@ -494,10 +494,30 @@ TEST_F(PageLoadTrackerTest, LargestImageIncorrectLoadTimings) {
       /*load_timing_info=*/
       std::make_unique<net::LoadTimingInfo>(load_timing_info));
 
-  // Set document receive_headers_start.
+  // Set main frame document receive_headers_start. This field should be set
+  // only once.
   const auto request_id = navigation_simulator->GetGlobalRequestID();
   tester()->SimulateLoadedResource(request_info, request_id);
 
+  // Simulate the invocation of PageLoadTracker::OnLoadedResource() again with
+  // a ttfb earlier than the image load start and a request destination that is
+  // not of type Document. This should not overwrite the
+  // receive_headers_start that is already set.
+  load_timing_info.receive_headers_start =
+      reference_time + base::Milliseconds(29);
+
+  ExtraRequestCompleteInfo request_info1(
+      /*final_url=*/url::SchemeHostPort(GURL(kTestUrl)),
+      /*remote_endpoint=*/net::IPEndPoint(),
+      /*frame_tree_node_id=*/-1, /*was_cached=*/false, /*raw_body_bytes=*/0,
+      /*original_network_content_length=*/0,
+      /*request_destination=*/network::mojom::RequestDestination::kFrame,
+      /*net_error=*/0,
+      /*load_timing_info=*/
+      std::make_unique<net::LoadTimingInfo>(load_timing_info));
+
+  tester()->SimulateLoadedResource(request_info1, request_id);
+
   // Set largest contentful paint timings.
   tester()->SimulateTimingUpdate(timing);