Notify page load metrics when the app enters the background.

chrome/browser/page_load_metrics/metrics_web_contents_observer.h

Index: chrome/browser/page_load_metrics/metrics_web_contents_observer.h
diff --git a/chrome/browser/page_load_metrics/metrics_web_contents_observer.h b/chrome/browser/page_load_metrics/metrics_web_contents_observer.h
index 16dece9bdfbadf3896b21917afe88a134fd20417..f1e5db3ee1d7ae6b0abe01af410b865e9b30ef07 100644
--- a/chrome/browser/page_load_metrics/metrics_web_contents_observer.h
+++ b/chrome/browser/page_load_metrics/metrics_web_contents_observer.h
@@ -139,6 +139,8 @@ class PageLoadTracker {
 
   void OnInputEvent(const blink::WebInputEvent& event);
 
+  void FlushMetricsOnAppEnterBackground();
+
   void NotifyClientRedirectTo(const PageLoadTracker& destination);
 
   // Returns true if the timing was successfully updated.
@@ -216,6 +218,10 @@ class PageLoadTracker {
   // metrics in DidFinishNavigation.
   bool did_stop_tracking_;
 
+  // Whether the application went into the background when this PageLoadTracker
+  // was active. This is a temporary boolean for UMA tracking.
+  bool app_entered_background_;
+
   // The navigation start in TimeTicks, not the wall time reported by Blink.
   const base::TimeTicks navigation_start_;
 
@@ -281,6 +287,12 @@ class MetricsWebContentsObserver
       std::unique_ptr<PageLoadMetricsEmbedderInterface> embedder_interface);
   ~MetricsWebContentsObserver() override;
 
+  // Returns all currently active MetricsWebContentsObservers. Callers should
+  // not hold on to the returned observers beyond the scope of the current
+  // function, as the MetricsWebContentsObservers may be destroyed as their
+  // associated WebContents are destroyed.
+  static std::vector<MetricsWebContentsObserver*> GetAllObservers();
+
   // content::WebContentsObserver implementation:
   bool OnMessageReceived(const IPC::Message& message,
                          content::RenderFrameHost* render_frame_host) override;
@@ -298,6 +310,8 @@ class MetricsWebContentsObserver
   void RenderViewHostChanged(content::RenderViewHost* old_host,
                              content::RenderViewHost* new_host) override;
 
+  void FlushMetricsOnAppEnterBackground();

[Charlie Harrison, 2016/07/28 14:45:28]

Do we need to include "flush metrics" in the name? Can observers just do what
they want with the knowledge that they app entered the background?

[Bryan McQuade, 2016/07/28 16:26:11]

Yeah - I debated this. Technically, this method is coupled to the metrics
subsystem's backgrounding logic. This callback is guaranteed to be called before
the metrics subsystem writes metrics out to disk as part of its backgrounding
logic. A generic OnAppEnterBackground wouldn't necessarily make this guarantee -
it might get called during backgrounding, but after the metrics subsystem has
finished flushing metrics, which wouldn't be the right timing for flushing
metrics. Similarly, if there's some other chrome subsystem that observers need
to interact with as part of backgrounding in the future, we'll probably need a
separate callback that's guaranteed to get called as part of the backgrounding
flow for that component, so we can be sure it's called e.g. before that other
subsystem is done persisting its data. Given all this, I decided it was better
to explicitly include 'FlushMetrics' in the name.

I added a comment to better explain.

[Charlie Harrison, 2016/07/28 16:38:04]

SGTM.

+
   // This getter function is required for testing.
   const PageLoadExtraInfo GetPageLoadExtraInfoForCommittedLoad();