Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/extensions/external_policy_loader.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_EXTENSIONS_EXTERNAL_POLICY_LOADER_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTERNAL_POLICY_LOADER_H_
 
 #include "chrome/browser/extensions/external_loader.h"
 
 #include "base/compiler_specific.h"
 #include "base/prefs/public/pref_change_registrar.h"
+#include "base/prefs/public/pref_observer.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 
 class Profile;
 
 namespace extensions {
 
 // A specialization of the ExternalProvider that uses
 // prefs::kExtensionInstallForceList to look up which external extensions are
 // registered.
 class ExternalPolicyLoader
     : public ExternalLoader,
-      public content::NotificationObserver {
+      public content::NotificationObserver,
+      public PrefObserver {
  public:
   explicit ExternalPolicyLoader(Profile* profile);
 
   // content::NotificationObserver implementation
   virtual void Observe(int type,
                        const content::NotificationSource& source,
                        const content::NotificationDetails& details) OVERRIDE;
 
+  // PrefObserver implementation
+  virtual void OnPreferenceChanged(PrefServiceBase* service,
+                                   const std::string& pref_name) OVERRIDE;
+
  protected:
   virtual void StartLoading() OVERRIDE;
 
  private:
   friend class base::RefCountedThreadSafe<ExternalLoader>;
 
   virtual ~ExternalPolicyLoader() {}
 
   PrefChangeRegistrar pref_change_registrar_;
   content::NotificationRegistrar notification_registrar_;
 
   Profile* profile_;
 
   DISALLOW_COPY_AND_ASSIGN(ExternalPolicyLoader);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTERNAL_POLICY_LOADER_H_