Replace more ObserverList with base::ObserverList.

extensions/browser/extension_registry.h

 // Copyright 2013 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_REGISTRY_H_
 #define EXTENSIONS_BROWSER_EXTENSION_REGISTRY_H_
 
 #include <string>
 
 #include "base/compiler_specific.h"
@@ skipping 165 matching lines @@
 
   // Extensions that are installed and blacklisted. Generally these shouldn't be
   // considered as installed by the extension platform: we only keep them around
   // so that if extensions are blacklisted by mistake they can easily be
   // un-blacklisted.
   ExtensionSet blacklisted_extensions_;
 
   // Extensions that are installed and blocked. Will never be loaded.
   ExtensionSet blocked_extensions_;
 
-  ObserverList<ExtensionRegistryObserver> observers_;
+  base::ObserverList<ExtensionRegistryObserver> observers_;
 
   content::BrowserContext* const browser_context_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionRegistry);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_REGISTRY_H_