Add the main frame routing ID to WebContentsDelegate, use it in BackgroundContents.

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 5478a378158c5dd4dcacfb8184555724693550dd..3304729711954f3ec73cc4980b247807cd917dc0 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -103,6 +103,10 @@ class WebContents : public PageNavigator,
     // If the opener is suppressed, then the new WebContents doesn't hold a
     // reference to its opener.
     bool opener_suppressed;
+
+    // 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;