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

content/browser/web_contents/web_contents_impl.h

Index: content/browser/web_contents/web_contents_impl.h
diff --git a/content/browser/web_contents/web_contents_impl.h b/content/browser/web_contents/web_contents_impl.h
index c325f5addd8c350093a74483218449653548a498..65c331d949478044c050bec17d638f96be78fdea 100644
--- a/content/browser/web_contents/web_contents_impl.h
+++ b/content/browser/web_contents/web_contents_impl.h
@@ -795,6 +795,11 @@ class CONTENT_EXPORT WebContentsImpl
   // Called when the WebContents gains or loses a persistent video.
   void SetHasPersistentVideo(bool value);
 
+  // Whether the WebContents has an active fullscreen video that is either using
+  // native or custom controls.

[Charlie Reis, 2017/02/23 00:18:08]

I think the description of "effectively fullscreen" from
media_web_contents_observer.h was clearer, and this is the comment more people
are likely to see.  Can you copy some of that here?

[mlamouri (slow - plz ping), 2017/02/23 13:34:19]

Done.

+  // |IsFullscreen| must return |true| when this method is called.
+  bool HasActiveEffectivelyFullscreenVideo() const;
+
 #if defined(OS_ANDROID)
   // Called by FindRequestManager when all of the find match rects are in.
   void NotifyFindMatchRectsReply(int version,