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 47decf4c6ea90c6e5f2cc716adf666cc9073014b..48fad2f9dce819cd705803d9c10fab63987fdedf 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -112,11 +112,15 @@ 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.

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

Maybe the documentation should probably say which process_id these route_ids are
for (it's the opener's process -- at least I pray it is -- right?).

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

Done, though it's actually for site_instance->GetProcess()->GetID().

+    int32 routing_id;
+    int32 main_frame_routing_id;
+    int32 main_frame_widget_routing_id;
+
+    // The surface ID of the main frame widget.

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

Add a comment saying when this must be provided (it's required when we have a
widget_routing_id, right?).

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

Done.

+    int32 surface_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