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

media/blink/webmediaplayer_impl.cc

Index: media/blink/webmediaplayer_impl.cc
diff --git a/media/blink/webmediaplayer_impl.cc b/media/blink/webmediaplayer_impl.cc
index f8c8d24f13ce728929938606f35f67c547e7243d..aec1ea699ee6effb6145b5264b8744a6cec21558 100644
--- a/media/blink/webmediaplayer_impl.cc
+++ b/media/blink/webmediaplayer_impl.cc
@@ -356,6 +356,7 @@ void WebMediaPlayerImpl::enteredFullscreen() {
   }
   if (observer_)
     observer_->OnEnteredFullscreen();
+  delegate_->SetIsEffectivelyFullscreen(delegate_id_, true);
 }
 
 void WebMediaPlayerImpl::exitedFullscreen() {
@@ -365,6 +366,7 @@ void WebMediaPlayerImpl::exitedFullscreen() {
     DisableOverlay();
   if (observer_)
     observer_->OnExitedFullscreen();
+  delegate_->SetIsEffectivelyFullscreen(delegate_id_, false);
 }
 
 void WebMediaPlayerImpl::becameDominantVisibleContent(bool isDominant) {