Replace more ObserverList with base::ObserverList.

extensions/browser/extension_prefs.h

 // Copyright 2014 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 EXTENSIONS_BROWSER_EXTENSION_PREFS_H_
 #define EXTENSIONS_BROWSER_EXTENSION_PREFS_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 670 matching lines @@
   ExtensionPrefValueMap* extension_pref_value_map_;
 
   // Contains all the logic for handling the order for various extension
   // properties.
   scoped_ptr<AppSorting> app_sorting_;
 
   scoped_ptr<TimeProvider> time_provider_;
 
   bool extensions_disabled_;
 
-  ObserverList<ExtensionPrefsObserver> observer_list_;
+  base::ObserverList<ExtensionPrefsObserver> observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionPrefs);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_PREFS_H_