Give the main frame a RenderWidget.

content/browser/frame_host/render_frame_host_impl.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_IMPL_H_
 #define CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_IMPL_H_
 
 #include <map>
 #include <vector>
 
@@ skipping 203 matching lines @@
   RenderViewHostImpl* render_view_host() { return render_view_host_; }
   RenderFrameHostDelegate* delegate() { return delegate_; }
   FrameTreeNode* frame_tree_node() { return frame_tree_node_; }
 
   // Returns this RenderFrameHost's loading state. This method is only used by
   // FrameTreeNode. The proper way to check whether a frame is loading is to
   // call FrameTreeNode::IsLoading.
   bool is_loading() const { return is_loading_; }
 
   // This returns the RenderFrameHost's owned RenderWidgetHost if it has one,
   // or else it returns nullptr.

[ncarter (slow), 2015/09/03 20:24:37]

I think these comments aren't quite clear enough to help you make the right
choice between between GetRenderWidgetHost() and GetRenderWidgetHostForFrame().
Some ideas:

- Maybe add TODOs to both functions, "unify these two functions once the main
frame doesn't have two widgets" or something?

- "Use GetRenderWidgetHost() when you depend on functionality that hasn't been
migrated from the RVH to the RWH" or something like that.

[dcheng, 2015/09/03 21:29:24]

Done.

   // If the RenderFrameHost is the page's main frame, this returns instead a
   // pointer to the RenderViewHost (which inherits RenderWidgetHost).
   RenderWidgetHostImpl* GetRenderWidgetHost();
 
+  // Similar to the above, but if the RenderFrameHost is the main frame, it
+  // returns the RenderWidgetHost owned by the main RenderFrameHost rather than
+  // the RenderViewHost.
+  RenderWidgetHostImpl* GetRenderWidgetHostForFrame() {
+    return render_widget_host_;
+  }
+
   // This returns the RenderWidgetHostView that can be used to control
   // focus and visibility for this frame.
   RenderWidgetHostView* GetView();
 
   // This function is called when this is a swapped out RenderFrameHost that
   // lives in the same process as the parent frame. The
   // |cross_process_frame_connector| allows the non-swapped-out
   // RenderFrameHost for a frame to communicate with the parent process
   // so that it may composite drawing data.
   //
@@ skipping 540 matching lines @@
 
   // NOTE: This must be the last member.
   base::WeakPtrFactory<RenderFrameHostImpl> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderFrameHostImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_IMPL_H_