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 9c71f71733df39c73b6201b77a2367f8e0916976..a75629709e6a7d7ae5e8891ef262fbb5bf3c537b 100644
--- a/content/common/view_messages.h
+++ b/content/common/view_messages.h
@@ -360,6 +360,14 @@ IPC_STRUCT_BEGIN(ViewHostMsg_CreateWindow_Params)
   IPC_STRUCT_MEMBER(std::vector<base::string16>, additional_features)
 IPC_STRUCT_END()
 
+IPC_STRUCT_BEGIN(ViewHostMsg_CreateWindow_Reply)
+  IPC_STRUCT_MEMBER(int32, route_id)

[dcheng, 2015/08/24 18:50:20]

I used the non _t versions to be consistent with what's already present in this
file.

+  IPC_STRUCT_MEMBER(int32, main_frame_route_id)
+  IPC_STRUCT_MEMBER(int32, main_frame_widget_route_id)
+  IPC_STRUCT_MEMBER(int32, surface_id)
+  IPC_STRUCT_MEMBER(int64, cloned_session_storage_namespace_id)
+IPC_STRUCT_END()

[ncarter (slow), 2015/08/24 20:55:35]

Can you add comments to these IPC_STRUCT_MEMBERS? In particular I'd like some
description to help differentiate the objects the various route_id's correspond
to.

[dcheng, 2015/09/03 16:33:09]

Done.

+
 IPC_STRUCT_BEGIN(ViewHostMsg_CreateWorker_Params)
   // URL for the worker script.
   IPC_STRUCT_MEMBER(GURL, url)
@@ -504,6 +512,9 @@ IPC_STRUCT_BEGIN(ViewMsg_New_Params)
   // The ID of the main frame hosted in the view.
   IPC_STRUCT_MEMBER(int32, main_frame_routing_id)
 
+  // The ID of the widget for the main frame.
+  IPC_STRUCT_MEMBER(int32, main_frame_widget_routing_id)
+
   // The ID of the rendering surface.
   IPC_STRUCT_MEMBER(int32, surface_id)
 
@@ -1001,12 +1012,9 @@ 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

[ncarter (slow), 2015/08/24 20:55:35]

This comment about "route_id" is stale.

[dcheng, 2015/09/03 16:33:09]

I've moved this comment to ViewHsotMsg_CreateWindow_Reply.

 // MSG_ROUTING_NONE, the view couldn't be created.
-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)

[dcheng, 2015/08/24 18:50:20]

IPCs can't have more than 4 out parameters. I wasn't able to find a good naming
convention for reply structs, so I just made something up

 
 // Similar to ViewHostMsg_CreateWindow, except used for sub-widgets, like
 // <select> dropdowns.  This message is sent to the WebContentsImpl that