Give the main frame a RenderWidget.

content/common/view_messages.h

Index: content/common/view_messages.h
diff --git a/content/common/view_messages.h b/content/common/view_messages.h
index a852758568aef5dfc5abceafb49fc7b20ca7e1ae..10638c9aeb33368cc4e5ec88d7493cb883cad809 100644
--- a/content/common/view_messages.h
+++ b/content/common/view_messages.h
@@ -361,6 +361,28 @@ IPC_STRUCT_BEGIN(ViewHostMsg_CreateWindow_Params)
   IPC_STRUCT_MEMBER(std::vector<base::string16>, additional_features)
 IPC_STRUCT_END()
 
+IPC_STRUCT_BEGIN(ViewHostMsg_CreateWindow_Reply)
+  // The ID of the view to be created. If the ID is MSG_ROUTING_NONE, then the
+  // view couldn't be created.
+  IPC_STRUCT_MEMBER(int32, route_id, MSG_ROUTING_NONE)
+
+  // The ID of the view's rendering surface.
+  // TODO(dcheng): Remove this, once RenderView is no longer a RenderWidget.
+  IPC_STRUCT_MEMBER(int32, surface_id)
+
+  // The ID of the main frame hosted in the view.
+  IPC_STRUCT_MEMBER(int32, main_frame_route_id, MSG_ROUTING_NONE)
+
+  // The ID of the widget for the main frame.
+  IPC_STRUCT_MEMBER(int32, main_frame_widget_route_id, MSG_ROUTING_NONE)
+
+  // The ID of the main frame widget's rendering surface.
+  IPC_STRUCT_MEMBER(int32, widget_surface_id)
+
+  // TODO(dcheng): No clue. This is kind of duplicated from ViewMsg_New_Params.
+  IPC_STRUCT_MEMBER(int64, cloned_session_storage_namespace_id)
+IPC_STRUCT_END()
+
 IPC_STRUCT_BEGIN(ViewHostMsg_CreateWorker_Params)
   // URL for the worker script.
   IPC_STRUCT_MEMBER(GURL, url)
@@ -500,20 +522,27 @@ IPC_STRUCT_BEGIN(ViewMsg_New_Params)
   IPC_STRUCT_MEMBER(content::WebPreferences, web_preferences)
 
   // The ID of the view to be created.
-  IPC_STRUCT_MEMBER(int32, view_id)
+  IPC_STRUCT_MEMBER(int32, view_id, MSG_ROUTING_NONE)
+
+  // The ID of the view's rendering surface.
+  // TODO(dcheng): Remove this, once RenderView is no longer a RenderWidget.
+  IPC_STRUCT_MEMBER(int32, view_surface_id)
 
   // The ID of the main frame hosted in the view.
-  IPC_STRUCT_MEMBER(int32, main_frame_routing_id)
+  IPC_STRUCT_MEMBER(int32, main_frame_routing_id, MSG_ROUTING_NONE)
 
-  // The ID of the rendering surface.
-  IPC_STRUCT_MEMBER(int32, surface_id)
+  // The ID of the widget for the main frame.
+  IPC_STRUCT_MEMBER(int32, main_frame_widget_routing_id, MSG_ROUTING_NONE)
+
+  // The ID of the main frame widget's rendering surface.
+  IPC_STRUCT_MEMBER(int32, widget_surface_id)
 
   // The session storage namespace ID this view should use.
   IPC_STRUCT_MEMBER(int64, session_storage_namespace_id)
 
   // The route ID of the opener RenderFrame or RenderFrameProxy, if we need to
   // set one (MSG_ROUTING_NONE otherwise).
-  IPC_STRUCT_MEMBER(int, opener_frame_route_id)
+  IPC_STRUCT_MEMBER(int, opener_frame_route_id, MSG_ROUTING_NONE)
 
   // Whether the RenderView should initially be swapped out.
   IPC_STRUCT_MEMBER(bool, swapped_out)
@@ -526,7 +555,7 @@ IPC_STRUCT_BEGIN(ViewMsg_New_Params)
 
   // The ID of the proxy object for the main frame in this view. It is only
   // used if |swapped_out| is true.
-  IPC_STRUCT_MEMBER(int32, proxy_routing_id)
+  IPC_STRUCT_MEMBER(int32, proxy_routing_id, MSG_ROUTING_NONE)
 
   // Whether the RenderView should initially be hidden.
   IPC_STRUCT_MEMBER(bool, hidden)
@@ -1002,12 +1031,9 @@ IPC_MESSAGE_ROUTED1(ViewHostMsg_SetNeedsBeginFrames,
 // Sent by the renderer when it is creating a new window.  The browser creates
 // a tab for it and responds with a ViewMsg_CreatingNew_ACK.  If route_id is
 // MSG_ROUTING_NONE, the view couldn't be created.

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

route_id -> |reply.route_id| ?

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

Doh, I thought I changed this one. Done.

-IPC_SYNC_MESSAGE_CONTROL1_4(ViewHostMsg_CreateWindow,
+IPC_SYNC_MESSAGE_CONTROL1_1(ViewHostMsg_CreateWindow,
                             ViewHostMsg_CreateWindow_Params,
-                            int /* route_id */,
-                            int /* main_frame_route_id */,
-                            int32 /* surface_id */,
-                            int64 /* cloned_session_storage_namespace_id */)
+                            ViewHostMsg_CreateWindow_Reply)
 
 // Similar to ViewHostMsg_CreateWindow, except used for sub-widgets, like
 // <select> dropdowns.  This message is sent to the WebContentsImpl that