Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/ui/alternate_error_tab_observer.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_UI_ALTERNATE_ERROR_TAB_OBSERVER_H_
 #define CHROME_BROWSER_UI_ALTERNATE_ERROR_TAB_OBSERVER_H_
 
 #include "base/prefs/public/pref_change_registrar.h"
+#include "base/prefs/public/pref_observer.h"
 #include "chrome/browser/prefs/pref_service.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "content/public/browser/web_contents_observer.h"
 #include "content/public/browser/web_contents_user_data.h"
 
 class Profile;
 
 // Per-tab class to implement alternate error page functionality.
 class AlternateErrorPageTabObserver
     : public content::WebContentsObserver,
       public content::NotificationObserver,
-      public content::WebContentsUserData<AlternateErrorPageTabObserver> {
+      public content::WebContentsUserData<AlternateErrorPageTabObserver>,
+      public PrefObserver {
  public:
   virtual ~AlternateErrorPageTabObserver();
 
   static void RegisterUserPrefs(PrefService* prefs);
 
  private:
   explicit AlternateErrorPageTabObserver(content::WebContents* web_contents);
   friend class content::WebContentsUserData<AlternateErrorPageTabObserver>;
 
   // content::WebContentsObserver overrides:
   virtual void RenderViewCreated(
       content::RenderViewHost* render_view_host) OVERRIDE;
 
   // content::NotificationObserver overrides:
   virtual void Observe(int type,
                        const content::NotificationSource& source,
                        const content::NotificationDetails& details) OVERRIDE;
 
+  // PrefObserver overrides:
+  virtual void OnPreferenceChanged(PrefServiceBase* service,
+                                   const std::string& pref_name) OVERRIDE;
+
   // Internal helpers ----------------------------------------------------------
 
   // Returns the server that can provide alternate error pages.  If the returned
   // URL is empty, the default error page built into WebKit will be used.
   GURL GetAlternateErrorPageURL() const;
 
   // Send the alternate error page URL to the renderer.
   void UpdateAlternateErrorPageURL(content::RenderViewHost* rvh);
 
   Profile* profile_;
   content::NotificationRegistrar registrar_;
   PrefChangeRegistrar pref_change_registrar_;
 
   DISALLOW_COPY_AND_ASSIGN(AlternateErrorPageTabObserver);
 };
 
 #endif  // CHROME_BROWSER_UI_ALTERNATE_ERROR_TAB_OBSERVER_H_