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

content/browser/tab_contents/tab_contents.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.h"
 
 #include <cmath>
 
 #include "base/command_line.h"
 #include "base/metrics/histogram.h"
@@ skipping 185 matching lines @@
       is_showing_before_unload_dialog_(false),
       opener_web_ui_type_(WebUI::kNoWebUI),
       closed_by_user_gesture_(false),
       minimum_zoom_percent_(
           static_cast<int>(WebKit::WebView::minTextSizeMultiplier * 100)),
       maximum_zoom_percent_(
           static_cast<int>(WebKit::WebView::maxTextSizeMultiplier * 100)),
       temporary_zoom_settings_(false),
       content_restrictions_(0),
       view_type_(content::VIEW_TYPE_TAB_CONTENTS) {
-
   render_manager_.Init(browser_context, site_instance, routing_id);
 
   // We have the initial size of the view be based on the size of the passed in
   // tab contents (normally a tab from the same window).
   view_->CreateView(base_tab_contents ?
       base_tab_contents->view()->GetContainerSize() : gfx::Size());
 
 #if defined(ENABLE_JAVA_BRIDGE)
   java_bridge_dispatcher_host_manager_.reset(
       new JavaBridgeDispatcherHostManager(this));
@@ skipping 1480 matching lines @@
 
   // Update the URL display.
   NotifyNavigationStateChanged(TabContents::INVALIDATE_URL);
 }
 
 void TabContents::DidChangeLoadProgress(double progress) {
   if (delegate())
     delegate()->LoadProgressChanged(progress);
 }
 
+void TabContents::DocumentAvailableInMainFrame(
+    RenderViewHost* render_view_host) {
+  FOR_EACH_OBSERVER(TabContentsObserver, observers_,
+                    DocumentAvailableInMainFrame());
+}
+
 void TabContents::DocumentOnLoadCompletedInMainFrame(
     RenderViewHost* render_view_host,
     int32 page_id) {
   content::NotificationService::current()->Notify(
       content::NOTIFICATION_LOAD_COMPLETED_MAIN_FRAME,
       content::Source<TabContents>(this),
       content::Details<int>(&page_id));
 }
 
 void TabContents::RequestOpenURL(const GURL& url,
@@ skipping 305 matching lines @@
 
 void TabContents::CreateViewAndSetSizeForRVH(RenderViewHost* rvh) {
   RenderWidgetHostView* rwh_view = view()->CreateViewForWidget(rvh);
   rwh_view->SetSize(view()->GetContainerSize());
 }
 
 bool TabContents::GotResponseToLockMouseRequest(bool allowed) {
   return render_view_host() ?
       render_view_host()->GotResponseToLockMouseRequest(allowed) : false;
 }