Buffer cross frame paint timing updates.

chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.cc

Index: chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.cc
diff --git a/chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.cc b/chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.cc
index bc777a07be34d4562fef85557e98d1068d3adab8..301b4fcbfaebad739d90e87a08f03b7d3adf852d 100644
--- a/chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.cc
+++ b/chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.cc
@@ -38,6 +38,13 @@ class TestPageLoadMetricsEmbedderInterface
     test_->RegisterObservers(tracker);
   }
 
+  std::unique_ptr<base::Timer> CreateTimer() override {
+    std::unique_ptr<PageLoadMetricsObserverTestHarness::WeakMockTimer> timer =
+        base::MakeUnique<PageLoadMetricsObserverTestHarness::WeakMockTimer>();
+    test_->SetMostRecentTimer(timer->AsWeakPtr());
+    return std::move(timer);
+  }
+
  private:
   PageLoadMetricsObserverTestHarness* test_;
 
@@ -146,6 +153,12 @@ void PageLoadMetricsObserverTestHarness::SimulateTimingAndMetadataUpdate(
     const mojom::PageLoadTiming& timing,
     const mojom::PageLoadMetadata& metadata) {
   observer_->OnTimingUpdated(web_contents()->GetMainFrame(), timing, metadata);
+  // If sending the timing update caused the PageLoadMetricsUpdateDispatcher to
+  // schedule a buffering timer, then fire it now so metrics are dispatched to
+  // observers.
+  base::MockTimer* mock_timer = GetMostRecentTimer();
+  if (mock_timer && mock_timer->IsRunning())
+    mock_timer->Fire();
 }
 
 void PageLoadMetricsObserverTestHarness::SimulateStartedResource(