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

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index e5560966a674c2924dbc9e3e8a76a259a0efe1dd..eaa73df32b1cb6dd06b988da8f4fa9461ce2e6fe 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -4222,6 +4222,10 @@ bool WebContentsImpl::HasPersistentVideo() const {
   return has_persistent_video_;
 }
 
+bool WebContentsImpl::HasActiveFullscreenDominantVideo() const {
+  return media_web_contents_observer_->HasActiveFullscreenDominantVideo();
+}
+
 bool WebContentsImpl::IsFocusedElementEditable() {
   RenderFrameHostImpl* frame = GetFocusedFrame();
   return frame && frame->has_focused_editable_element();