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

chrome/browser/extensions/extension_host_mac.h

 // 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_EXTENSION_HOST_MAC_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_HOST_MAC_H_
 #pragma once
 
 #include "chrome/browser/extensions/extension_host.h"
 
 class RenderWidgetHostView;
 
+// TODO(mpcomplete): I don't know what this does or if it is needed anymore,
+// now that ExtensionHost is restructured to rely on TabContents.
 class ExtensionHostMac : public ExtensionHost {
  public:
   ExtensionHostMac(const Extension* extension, SiteInstance* site_instance,
                    const GURL& url, content::ViewType host_type) :
       ExtensionHost(extension, site_instance, url, host_type) {}
   virtual ~ExtensionHostMac();
- protected:
-  virtual RenderWidgetHostView* CreateNewWidgetInternal(
-      int route_id,
-      WebKit::WebPopupType popup_type);
-  virtual void ShowCreatedWidgetInternal(RenderWidgetHostView* widget_host_view,
-                                         const gfx::Rect& initial_pos);
  private:
   virtual void UnhandledKeyboardEvent(const NativeWebKeyboardEvent& event);
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionHostMac);
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_HOST_MAC_H_