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

chrome/browser/tab_contents/render_view_host_delegate_helper.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 "chrome/browser/tab_contents/render_view_host_delegate_helper.h"
 
 #include <string>
 
 #include "base/command_line.h"
 #include "base/string_util.h"
@@ skipping 531 matching lines @@
     web_prefs.loads_images_automatically = true;
     web_prefs.javascript_enabled = true;
   }
 
   web_prefs.is_online = !net::NetworkChangeNotifier::IsOffline();
 
   ExtensionService* service = profile->GetExtensionService();
   if (service) {
     const Extension* extension =
         service->GetExtensionByURL(rvh->site_instance()->site());
-    extension_webkit_preferences::SetPreferences(&web_prefs, extension);
+    extension_webkit_preferences::SetPreferences(
+        extension, rvh->delegate()->GetRenderViewType(), &web_prefs);
   }
 
   if (rvh->delegate()->GetRenderViewType() == chrome::VIEW_TYPE_NOTIFICATION) {
     web_prefs.allow_scripts_to_close_windows = true;
   } else if (rvh->delegate()->GetRenderViewType() ==
              chrome::VIEW_TYPE_BACKGROUND_CONTENTS) {
     // Disable all kinds of acceleration for background pages.
     // See http://crbug.com/96005 and http://crbug.com/96006
     web_prefs.force_compositing_mode = false;
     web_prefs.accelerated_compositing_enabled = false;
@@ skipping 16 matching lines @@
   DictionaryValue* inspector_settings = update.Get();
   inspector_settings->SetWithoutPathExpansion(key,
                                               Value::CreateStringValue(value));
 }
 
 void RenderViewHostDelegateHelper::ClearInspectorSettings(
     content::BrowserContext* browser_context) {
   Profile::FromBrowserContext(browser_context)->GetPrefs()->
       ClearPref(prefs::kWebKitInspectorSettings);
 }