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 bea9a92a34c679b3dc60456d9d76b9d03d7d1536..e7f16704a9f98c5de1c47bbe406b642c71455954 100644
--- a/media/blink/webmediaplayer_impl.cc
+++ b/media/blink/webmediaplayer_impl.cc
@@ -357,6 +357,7 @@ void WebMediaPlayerImpl::enteredFullscreen() {
   }
   if (observer_)
     observer_->OnEnteredFullscreen();
+  delegate_->SetIsFullscreen(delegate_id_, true);
 }
 
 void WebMediaPlayerImpl::exitedFullscreen() {
@@ -366,6 +367,7 @@ void WebMediaPlayerImpl::exitedFullscreen() {
     DisableOverlay();
   if (observer_)
     observer_->OnExitedFullscreen();
+  delegate_->SetIsFullscreen(delegate_id_, false);
 }
 
 void WebMediaPlayerImpl::becameDominantVisibleContent(bool isDominant) {