[Prerender] first contentful paint histograms.

chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h b/chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h
index 7e55fdbda276d1232cd28d8e5a8004b41173ad44..bcebe45350d7db4e8fccecd50f75c8c8ce2d463e 100644
--- a/chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h
@@ -36,8 +36,12 @@ class NoStatePrefetchPageLoadMetricsObserver
   void OnFirstContentfulPaint(
       const page_load_metrics::PageLoadTiming& timing,
       const page_load_metrics::PageLoadExtraInfo& extra_info) override;
+  ObservePolicy OnHidden(
+      const page_load_metrics::PageLoadTiming& timing,
+      const page_load_metrics::PageLoadExtraInfo& extra_info) override;
 
   bool is_no_store_;  // True if the main resource has a "no-store" HTTP header.
+  bool was_hidden_;   // The page went to background while rendering.

[pasko, 2016/12/20 19:33:34]

I am wondering whether there is a mechanism to detect that chrome itself went to
background while prerender was in progress. Do we want to add these as .Hidden.?

[mattcary, 2016/12/21 17:08:58]

No idea. I'll ping Bryan.

[Bryan McQuade, 2016/12/21 17:56:31]

OnHidden is a per-tab signal.

We don't really have an overall browser signal indicating whether the browser
gets foregrounded / backgrounded, but we could add one.

We do have the FlushMetricsOnAppEnterBackground callback, which is invoked on
Android when the app goes into the background. This is mostly provided so we can
persist any buffered metrics in this callback, since Chrome may be killed at any
point without subsequent notification after this method is invoked (this gets
invoked as part of the Activity.onPause flow when we don't have any more
activities in the foreground).

[pasko, 2016/12/21 18:39:08]

To me this suggests that we do not worry about onPause in metrics for Android,
hence we probably should not worry here for Prerender TTFCP. Is that a close
enough interpretation?

[mattcary, 2016/12/22 10:49:07]

sgtm

   prerender::PrerenderManager* const prerender_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(NoStatePrefetchPageLoadMetricsObserver);