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 0fa7375bcc2d696445a008bc170d00aff3074539..fe7624a267325e7b919af66aaef8a27865094036 100644
--- a/media/blink/webmediaplayer_impl.cc
+++ b/media/blink/webmediaplayer_impl.cc
@@ -354,6 +354,7 @@ void WebMediaPlayerImpl::enteredFullscreen() {
   }
   if (observer_)
     observer_->OnEnteredFullscreen();
+  delegate_->SetIsFullscreen(delegate_id_, true);
 }
 
 void WebMediaPlayerImpl::exitedFullscreen() {
@@ -363,11 +364,13 @@ void WebMediaPlayerImpl::exitedFullscreen() {
     DisableOverlay();
   if (observer_)
     observer_->OnExitedFullscreen();
+  delegate_->SetIsFullscreen(delegate_id_, false);
 }
 
 void WebMediaPlayerImpl::becameDominantVisibleContent(bool isDominant) {
   if (observer_)
     observer_->OnBecameDominantVisibleContent(isDominant);
+  delegate_->SetIsDominant(delegate_id_, isDominant);
 }
 
 void WebMediaPlayerImpl::DoLoad(LoadType load_type,