Switch WebContentsUserData to be a friended base.

chrome/browser/sessions/session_tab_helper.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_SESSIONS_SESSION_TAB_HELPER_H_
 #define CHROME_BROWSER_SESSIONS_SESSION_TAB_HELPER_H_
 
 #include "base/basictypes.h"
 #include "chrome/browser/sessions/session_id.h"
 #include "chrome/browser/tab_contents/web_contents_user_data.h"
 #include "content/public/browser/web_contents_observer.h"
 
 // This class keeps the extension API's windowID up-to-date with the current
 // window of the tab.
 class SessionTabHelper : public content::WebContentsObserver,
                          public WebContentsUserData<SessionTabHelper> {
  public:
-  explicit SessionTabHelper(content::WebContents* contents);
   virtual ~SessionTabHelper();
-  static int kUserDataKey;
 
   // Returns the identifier used by session restore for this tab.
   const SessionID& session_id() const { return session_id_; }
 
   // Identifier of the window the tab is in.
   void SetWindowID(const SessionID& id);
   const SessionID& window_id() const { return window_id_; }
 
   // content::WebContentsObserver:
   virtual void RenderViewCreated(
       content::RenderViewHost* render_view_host) OVERRIDE;
   virtual void UserAgentOverrideSet(const std::string& user_agent) OVERRIDE;
 
  private:
+  explicit SessionTabHelper(content::WebContents* contents);
+  static int kUserDataKey;
+  friend class WebContentsUserData<SessionTabHelper>;
+
   // Unique identifier of the tab for session restore. This id is only unique
   // within the current session, and is not guaranteed to be unique across
   // sessions.
   const SessionID session_id_;
 
   // Unique identifier of the window the tab is in.
   SessionID window_id_;
 
   DISALLOW_COPY_AND_ASSIGN(SessionTabHelper);
 };
 
 #endif  // CHROME_BROWSER_SESSIONS_SESSION_TAB_HELPER_H_