Replace more ObserverList with base::ObserverList.

ui/aura/env.h

 // Copyright (c) 2012 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_AURA_ENV_H_
 #define UI_AURA_ENV_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list.h"
 #include "base/supports_user_data.h"
@@ skipping 75 matching lines @@
 
   // Invoked by WindowTreeHost when it is activated. Notifies observers.
   void NotifyHostActivated(WindowTreeHost* host);
 
   // Overridden from ui::EventTarget:
   bool CanAcceptEvent(const ui::Event& event) override;
   ui::EventTarget* GetParentTarget() override;
   scoped_ptr<ui::EventTargetIterator> GetChildIterator() const override;
   ui::EventTargeter* GetEventTargeter() override;
 
-  ObserverList<EnvObserver> observers_;
+  base::ObserverList<EnvObserver> observers_;
 
   int mouse_button_flags_;
   // Location of last mouse event, in screen coordinates.
   gfx::Point last_mouse_location_;
   bool is_touch_down_;
 
   scoped_ptr<InputStateLookup> input_state_lookup_;
   scoped_ptr<ui::PlatformEventSource> event_source_;
 
   ui::ContextFactory* context_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(Env);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_ENV_H_