Use observer pattern instead of sniffing SwapCompositorFrame IPC

content/public/browser/web_contents_observer.h

Index: content/public/browser/web_contents_observer.h
diff --git a/content/public/browser/web_contents_observer.h b/content/public/browser/web_contents_observer.h
index 3de52be6ddc99655bb6148d9c25f1379df5ec10b..92b3f530c11d0dce6476314557cf1053c3cd770a 100644
--- a/content/public/browser/web_contents_observer.h
+++ b/content/public/browser/web_contents_observer.h
@@ -423,6 +423,9 @@ class CONTENT_EXPORT WebContentsObserver : public IPC::Listener,
   // Notification that |contents| has gained focus.
   virtual void OnWebContentsFocused() {}
 
+  // Notifes that a CompositorFrame was received from the renderer.
+  virtual void DidReceiveCompositorFrame() {}
+
   // IPC::Listener implementation.
   bool OnMessageReceived(const IPC::Message& message) override;