Rework FocusManager as FocusClient.

ui/aura/client/focus_change_observer.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_FOCUS_CHANGE_OBSERVER_H_
-#define UI_AURA_FOCUS_CHANGE_OBSERVER_H_
+#ifndef UI_AURA_CLIENT_FOCUS_CHANGE_OBSERVER_H_
+#define UI_AURA_CLIENT_FOCUS_CHANGE_OBSERVER_H_
 
 #include "ui/aura/aura_export.h"
 
 namespace aura {
+class Window;
+namespace client {
 
-class Window;
-
+// TODO(beng): this interface will be OBSOLETE by FocusChangeEvent.
 class AURA_EXPORT FocusChangeObserver {
  public:
   // Called when |window| gains focus.
   virtual void OnWindowFocused(Window* window) = 0;
 
  protected:
   virtual ~FocusChangeObserver() {}
 };
 
+}  // namespace client
 }  // namespace aura
 
-#endif  // UI_AURA_FOCUS_CHANGE_OBSERVER_H_
+#endif  // UI_AURA_CLIENT_FOCUS_CHANGE_OBSERVER_H_