Replace more ObserverList with base::ObserverList.

ios/web/active_state_manager_impl.h

 // Copyright 2015 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 IOS_WEB_ACTIVE_STATE_MANAGER_IMPL_H_
 #define IOS_WEB_ACTIVE_STATE_MANAGER_IMPL_H_
 
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "base/supports_user_data.h"
@@ skipping 30 matching lines @@
   // than once. The caller retains the ownership of the observer object.
   void AddObserver(Observer* obs);
   // Removes an observer.
   void RemoveObserver(Observer* obs);
 
  private:
   BrowserState* browser_state_;  // weak, owns this object.
   // true if the ActiveStateManager is active.
   bool active_;
   // The list of observers.
-  ObserverList<Observer> observer_list_;
+  base::ObserverList<Observer> observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(ActiveStateManagerImpl);
 };
 
 }  // namespace web
 
 #endif  // IOS_WEB_ACTIVE_STATE_MANAGER_IMPL_H_