Give the main frame a RenderWidget.

content/public/test/mock_render_thread.h

Index: content/public/test/mock_render_thread.h
diff --git a/content/public/test/mock_render_thread.h b/content/public/test/mock_render_thread.h
index b5e1550dea5c60d9c2b337af0456ddf5dd11c8f6..f60819ea6dc501b347b868492c0d57b07c601b54 100644
--- a/content/public/test/mock_render_thread.h
+++ b/content/public/test/mock_render_thread.h
@@ -15,6 +15,7 @@
 #include "third_party/WebKit/public/web/WebPopupType.h"
 
 struct ViewHostMsg_CreateWindow_Params;
+struct ViewHostMsg_CreateWindow_Reply;
 
 namespace IPC {
 class MessageFilter;
@@ -126,12 +127,8 @@ class MockRenderThread : public RenderThread {
   // The View expects to be returned a valid route_id different from its own.

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

|route_id| comment is stale.

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

Done.

   // We do not keep track of the newly created widget in MockRenderThread,
   // so it must be cleaned up on its own.
-  void OnCreateWindow(
-    const ViewHostMsg_CreateWindow_Params& params,
-    int* route_id,
-    int* main_frame_route_id,
-    int* surface_id,
-    int64* cloned_session_storage_namespace_id);
+  void OnCreateWindow(const ViewHostMsg_CreateWindow_Params& params,
+                      ViewHostMsg_CreateWindow_Reply* reply);
 
   // The Frame expects to be returned a valid route_id different from its own.
   void OnCreateChildFrame(int new_frame_routing_id,