Give the main frame a RenderWidget.

content/public/browser/web_contents.h

Index: content/public/browser/web_contents.h
diff --git a/content/public/browser/web_contents.h b/content/public/browser/web_contents.h
index 795bec5a678da5d77c5485d6e4a16b73063ecbd9..46918e51b4d57c401f8fffebcfab8e6913f1cb1b 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -113,11 +113,13 @@ class WebContents : public PageNavigator,
     // (e.g., for blocked popups).
     bool created_with_opener;
 
-    // The routing ids of the RenderView and of the main RenderFrame. Either
-    // both must be provided, or both must be MSG_ROUTING_NONE to have the
-    // WebContents make the assignment.
-    int routing_id;
-    int main_frame_routing_id;
+    // The routing ids of the RenderView, main RenderFrame, and the widget for
+    // the main RenderFrame. Either all routing IDs must be provided or all must
+    // be MSG_ROUTING_NONE to have WebContents make the assignment. If provided,
+    // these routing IDs are associated with |site_instance->GetProcess()|.
+    int32_t routing_id;
+    int32_t main_frame_routing_id;
+    int32_t main_frame_widget_routing_id;
 
     // The name of the top-level frame of the new window. It is non-empty
     // when creating a named window (e.g. <a target="foo"> or