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 244b5d4922c835be697dc75cfa18afcd4ba50f94..307e3fc64903bfe5bd14401e86a16e665fcc792a 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -4255,6 +4255,10 @@ bool WebContentsImpl::HasPersistentVideo() const {
   return has_persistent_video_;
 }
 
+bool WebContentsImpl::HasActiveEffectivelyFullscreenVideo() const {
+  return media_web_contents_observer_->HasActiveEffectivelyFullscreenVideo();
+}
+
 bool WebContentsImpl::IsFocusedElementEditable() {
   RenderFrameHostImpl* frame = GetFocusedFrame();
   return frame && frame->has_focused_editable_element();