Don't call RecordTimingMetrics in UKM observer's OnHidden.

Instead wait until OnComplete or app background to report these metrics,
to capture LCP reports that arrive after tab hide due to buffering
(go/plm-flushing).  The UMA observer already does this.

Bug: 1069023
Change-Id: Idfc061eb0b9e1694443b3468da927211e47110c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575002
Commit-Queue: Steve Kobes <skobes@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836176}

chrome/browser/page_load_metrics/observers/core/ukm_page_load_metrics_observer.h

@@ -141,9 +141,7 @@ class UkmPageLoadMetricsObserver
   void RecordRendererUsageMetrics();
 
   // Adds main resource timing metrics to |builder|.
-  void ReportMainResourceTimingMetrics(
-      const page_load_metrics::mojom::PageLoadTiming& timing,
-      ukm::builders::PageLoad* builder);
+  void ReportMainResourceTimingMetrics(ukm::builders::PageLoad& builder);
 
   void ReportLayoutStability();
 
@@ -185,6 +183,10 @@ class UkmPageLoadMetricsObserver
       const page_load_metrics::mojom::PageLoadTiming* timing,
       base::TimeTicks page_end_time);
 
+  // Records a score from the SiteEngagementService. Called when the page
+  // becomes hidden, or at the end of the session if the page is never hidden.
+  void RecordSiteEngagement() const;
+
   // Guaranteed to be non-null during the lifetime of |this|.
   network::NetworkQualityTracker* network_quality_tracker_;
 

chrome/browser/page_load_metrics/observers/core/ukm_page_load_metrics_observer_unittest.cc

@@ -1839,6 +1839,34 @@ TEST_F(UkmPageLoadMetricsObserverTest, FCPHiddenWhileFlushing) {
   }
 }
 
+TEST_F(UkmPageLoadMetricsObserverTest, LCPHiddenWhileFlushing) {
+  NavigateAndCommit(GURL(kTestUrl1));
+
+  // Simulate hiding the tab.
+  web_contents()->WasHidden();
+  base::TimeDelta background_time =
+      *tester()->GetDelegateForCommittedLoad().GetFirstBackgroundTime();
+
+  page_load_metrics::mojom::PageLoadTiming timing;
+  page_load_metrics::InitPageLoadTimingForTest(&timing);
+  timing.navigation_start = base::Time::FromDoubleT(1);
+  timing.paint_timing->largest_contentful_paint->largest_image_paint =
+      background_time;
+  timing.paint_timing->largest_contentful_paint->largest_image_paint_size = 50u;
+  PopulateExperimentalLCP(timing.paint_timing);
+  PopulateRequiredTimingFields(&timing);
+
+  // Simulate LCP at the same time as the hide (but reported after).
+  tester()->SimulateTimingUpdate(timing);
+
+  // Simulate closing the tab.
+  DeleteContents();
+
+  // Check that we reported the LCP UKM.
+  TestLCP(background_time.InMilliseconds(), LargestContentType::kImage,
+          true /* test_main_frame */);
+}
+
 class TestOfflinePreviewsUkmPageLoadMetricsObserver
     : public UkmPageLoadMetricsObserver {
  public: