Switch WebContentsUserData to be a friended base.

chrome/browser/tab_contents/web_contents_user_data.h

 // Copyright (c) 2012 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_TAB_CONTENTS_WEB_CONTENTS_USER_DATA_H_
 #define CHROME_BROWSER_TAB_CONTENTS_WEB_CONTENTS_USER_DATA_H_
 
 #include "base/supports_user_data.h"
 #include "content/public/browser/web_contents.h"
 
 // A base class for classes attached to, and scoped to, the lifetime of a
 // WebContents. For example:
 //
 // --- in foo_tab_helper.h ---
 // class FooTabHelper : public WebContentsUserData<FooTabHelper> {
 //  public:
+//   virtual ~FooTabHelper();
+//   // ... more public stuff here ...
+//  private:
 //   explicit FooTabHelper(content::WebContents* contents);
-//   virtual ~FooTabHelper();
 //   static int kUserDataKey;
-//  // ... more stuff here ...
+//   friend class WebContentsUserData<FooTabHelper>;
+//   // ... more private stuff here ...
 // }
 // --- in foo_tab_helper.cc ---
 // int FooTabHelper::kUserDataKey;
 //
 template <typename T>
 class WebContentsUserData : public base::SupportsUserData::Data {
  public:
   // Creates an object of type T, and attaches it to the specified WebContents.
   // If an instance is already attached, does nothing.
   static void CreateForWebContents(content::WebContents* contents) {
     if (!FromWebContents(contents))
       contents->SetUserData(&T::kUserDataKey, new T(contents));
   }
 
   // Retrieves the instance of type T that was attached to the specified
   // WebContents (via CreateForWebContents above) and returns it. If no instance
   // of the type was attached, returns NULL.
   static T* FromWebContents(content::WebContents* contents) {
     return static_cast<T*>(contents->GetUserData(&T::kUserDataKey));
   }
   static const T* FromWebContents(const content::WebContents* contents) {
     return static_cast<const T*>(contents->GetUserData(&T::kUserDataKey));
   }
 };
 
 #endif  // CHROME_BROWSER_TAB_CONTENTS_WEB_CONTENTS_USER_DATA_H_