Have ExtensionHost use TabContents instead of RenderViewHost. Try #3.

content/browser/tab_contents/tab_contents_observer.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/tab_contents/tab_contents_observer.h"
 
 #include "content/browser/renderer_host/render_view_host.h"
 #include "content/browser/tab_contents/navigation_details.h"
 #include "content/browser/tab_contents/tab_contents.h"
 
@@ skipping 45 matching lines @@
 }
 
 void TabContentsObserver::DidFailProvisionalLoad(
     int64 frame_id,
     bool is_main_frame,
     const GURL& validated_url,
     int error_code,
     const string16& error_description) {
 }
 
+void TabContentsObserver::DocumentAvailableInMainFrame() {
+}
+
 void TabContentsObserver::DocumentLoadedInFrame(int64 frame_id) {
 }
 
 void TabContentsObserver::DidFinishLoad(int64 frame_id) {
 }
 
 void TabContentsObserver::DidGetUserGesture() {
 }
 
 void TabContentsObserver::DidGetIgnoredUIEvent() {
@@ skipping 77 matching lines @@
 }
 
 void TabContentsObserver::TabContentsDestroyed() {
   // Do cleanup so that 'this' can safely be deleted from
   // TabContentsDestroyed.
   tab_contents_->RemoveObserver(this);
   TabContents* tab = tab_contents_;
   tab_contents_ = NULL;
   TabContentsDestroyed(tab);
 }