Replace more ObserverList with base::ObserverList.

ui/wm/core/default_activation_client.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 UI_WM_CORE_DEFAULT_ACTIVATION_CLIENT_H_
 #define UI_WM_CORE_DEFAULT_ACTIVATION_CLIENT_H_
 
 #include <vector>
 
 #include "base/compiler_specific.h"
@@ skipping 42 matching lines @@
 
   // This class explicitly does NOT store the active window in a window property
   // to make sure that ActivationChangeObserver is not treated as part of the
   // aura API. Assumptions to that end will cause tests that use this client to
   // fail.
   std::vector<aura::Window*> active_windows_;
 
   // The window which was active before the currently active one.
   aura::Window* last_active_;
 
-  ObserverList<aura::client::ActivationChangeObserver> observers_;
+  base::ObserverList<aura::client::ActivationChangeObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(DefaultActivationClient);
 };
 
 }  // namespace wm
 
 #endif  // UI_WM_CORE_DEFAULT_ACTIVATION_CLIENT_H_