[Extensions Mac] Implement developer mode warning on mac

chrome/browser/extensions/extension_message_bubble.h

 // Copyright (c) 2013 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_MESSAGE_BUBBLE_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_MESSAGE_BUBBLE_H_
 
 #include "base/bind.h"
 
 class Browser;
 
 namespace extensions {
 
-// The interface between the SuspiciousExtensionBubble bubble and its
-// controller.
+// The interface between an extension message bubble's view and its
+// corresponding ExtensionMessageBubbleController.
 class ExtensionMessageBubble {
  public:
   // Instruct the bubble to appear.
   virtual void Show() = 0;
+
+ protected:
+  virtual ~ExtensionMessageBubble() {}
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_MESSAGE_BUBBLE_H_