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..4b3d28ffce61d2204e2a8120e90c23d185706ae2 100644
--- a/media/blink/webmediaplayer_delegate.h
+++ b/media/blink/webmediaplayer_delegate.h
@@ -116,6 +116,11 @@ 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.
+  virtual void SetIsEffectivelyFullscreen(int player_id,
+                                          bool is_fullscreen) = 0;
+
  protected:
   WebMediaPlayerDelegate() = default;
   virtual ~WebMediaPlayerDelegate() = default;