Provide a WebContents API to discover the playback of a fullscreen video.

media/blink/webmediaplayer_delegate.h

Index: media/blink/webmediaplayer_delegate.h
diff --git a/media/blink/webmediaplayer_delegate.h b/media/blink/webmediaplayer_delegate.h
index 0795c8d18aa0f372dfc2473d34b3579d0243dbfa..11ccc6f3e719a6689228bcf2e67fb0f20ab29714 100644
--- a/media/blink/webmediaplayer_delegate.h
+++ b/media/blink/webmediaplayer_delegate.h
@@ -116,6 +116,10 @@ class WebMediaPlayerDelegate {
   // called and returned |true|.
   virtual bool IsStale(int player_id) = 0;
 
+  // Notifies the delegate that the player has entered fullscreen. This does not
+  // differentiate native controls fullscreen and custom controls fullscreen.

[liberato (no reviews please), 2017/02/22 18:28:20]

i like these new semantics better.  might want to include 'Effectively' here
too, just so that it matches the observer.

[mlamouri (slow - plz ping), 2017/02/22 19:23:26]

I agree. Updated :)

+  virtual void SetIsFullscreen(int player_id, bool is_fullscreen) = 0;
+
  protected:
   WebMediaPlayerDelegate() = default;
   virtual ~WebMediaPlayerDelegate() = default;