Always use the WebFrameWidget when attaching the root graphics

third_party/WebKit/public/web/WebFrameWidget.h

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 19 matching lines @@
 
 #ifndef WebFrameWidget_h
 #define WebFrameWidget_h
 
 #include "../platform/WebCommon.h"
 #include "../platform/WebPageVisibilityState.h"
 #include "public/web/WebWidget.h"
 
 namespace blink {
 
+class CompositorAnimationTimeline;
 class CompositorProxyClient;
+class GraphicsLayer;
 class WebLocalFrame;
 class WebView;
 class WebWidgetClient;
 
 class WebFrameWidget : public WebWidget {
 public:
     BLINK_EXPORT static WebFrameWidget* create(WebWidgetClient*, WebLocalFrame*);
     // Creates a frame widget for a WebView. Temporary helper to help transition
     // away from WebView inheriting WebWidget.
     // TODO(dcheng): Remove once transition is complete.
@@ skipping 21 matching lines @@
     virtual void setBaseBackgroundColor(WebColor) = 0;
 
     // TODO(dcheng): Temporary: there should only be one WebFrameWidget
     // implementation but the Blink API is currently in a transition state.
     // See https://goo.gl/7yVrnb. These methods should only be used inside
     // blink.
     virtual bool forSubframe() const { return true; }
     virtual void scheduleAnimation() = 0;
     virtual CompositorProxyClient* createCompositorProxyClient() = 0;
     virtual WebWidgetClient* client() const = 0;
+    virtual void setRootGraphicsLayer(GraphicsLayer*) = 0;

[dcheng, 2016/07/04 07:34:36]

Hmm, can we move the Blink-internal methods to their own section with a comment?
I'll clean it up in a followup CL.

[lfg, 2016/07/04 17:46:13]

Done.

+    virtual void attachCompositorAnimationTimeline(CompositorAnimationTimeline*) = 0;
+    virtual void detachCompositorAnimationTimeline(CompositorAnimationTimeline*) = 0;
 };
 
 } // namespace blink
 
 #endif