views: Extract Widget observer into its own header file.

ash/launcher/overflow_bubble.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 ASH_LAUNCHER_OVERFLOW_BUBBLE_H_
 #define ASH_LAUNCHER_OVERFLOW_BUBBLE_H_
 
 #include "ash/wm/shelf_types.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
-#include "ui/views/widget/widget.h"
+#include "ui/views/widget/widget_observer.h"
 
 namespace views {
 class View;
 }
 
 namespace ash {
 
 class LauncherDelegate;
 class LauncherModel;
 
 namespace internal {
 
 class LauncherView;
 
 // OverflowBubble displays an overflow bubble.
-class OverflowBubble : public views::Widget::Observer {
+class OverflowBubble : public views::WidgetObserver {
  public:
   OverflowBubble();
   virtual ~OverflowBubble();
 
   void Show(LauncherDelegate* delegate,
             LauncherModel* model,
             views::View* anchor,
             ShelfAlignment shelf_alignment,
             int overflow_start_index);
 
   void Hide();
 
   bool IsShowing() const { return !!bubble_; }
 
  private:
-  // views::Widget::Observer overrides:
+  // Overridden from views::WidgetObserver:
   virtual void OnWidgetClosing(views::Widget* widget) OVERRIDE;
 
   views::View* bubble_;  // Owned by views hierarchy.
 
   DISALLOW_COPY_AND_ASSIGN(OverflowBubble);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_LAUNCHER_OVERFLOW_BUBBLE_H_