Have ExtensionHost use TabContents instead of RenderViewHost. Try #2. The first

chrome/browser/ui/cocoa/extensions/extension_view_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_UI_COCOA_EXTENSIONS_EXTENSION_VIEW_MAC_H_
 #define CHROME_BROWSER_UI_COCOA_EXTENSIONS_EXTENSION_VIEW_MAC_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/gfx/size.h"
 
 class Browser;
 class ExtensionHost;
 class RenderViewHost;
-class RenderWidgetHostViewMac;
 class SkBitmap;
 
 // This class represents extension views. An extension view internally contains
 // a bridge to an extension process, which draws to the extension view's
 // native view object through IPC.
 class ExtensionViewMac {
  public:
   ExtensionViewMac(ExtensionHost* extension_host, Browser* browser);
   ~ExtensionViewMac();
 
@@ skipping 32 matching lines @@
 
  private:
   RenderViewHost* render_view_host() const;
 
   void CreateWidgetHostView();
 
   Browser* browser_;  // weak
 
   ExtensionHost* extension_host_;  // weak
 
-  // Created by us, but owned by its |native_view()|. We |release| the
-  // rwhv's native view in our destructor, effectively freeing this.
-  RenderWidgetHostViewMac* render_widget_host_view_;
-
   // The background the view should have once it is initialized. This is set
   // when the view has a custom background, but hasn't been initialized yet.
   SkBitmap pending_background_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionViewMac);
 };
 
 #endif  // CHROME_BROWSER_UI_COCOA_EXTENSIONS_EXTENSION_VIEW_MAC_H_