Replace more ObserverList with base::ObserverList.

extensions/browser/extension_host.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_HOST_H_
 #define EXTENSIONS_BROWSER_EXTENSION_HOST_H_
 
 #include <set>
 #include <string>
 
@@ skipping 191 matching lines @@
   // used to measure the responsiveness of UI. For example, it's important to
   // keep this as low as possible for popups. Contrast this to |load_start_|,
   // for which a low value does not necessarily mean a responsive UI, as
   // ExtensionHosts may sit in an ExtensionHostQueue for a long time.
   base::ElapsedTimer create_start_;
 
   // Measures how long since the initial URL started loading. This timer is
   // started only once the ExtensionHost has exited the ExtensionHostQueue.
   scoped_ptr<base::ElapsedTimer> load_start_;
 
-  ObserverList<ExtensionHostObserver> observer_list_;
-  ObserverList<DeferredStartRenderHostObserver>
+  base::ObserverList<ExtensionHostObserver> observer_list_;
+  base::ObserverList<DeferredStartRenderHostObserver>
       deferred_start_render_host_observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionHost);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_HOST_H_