views: Extract Widget observer into its own header file.

ash/system/tray/system_tray_bubble.cc

 // 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.
 
 #include "ash/system/tray/system_tray_bubble.h"
 
 #include "ash/shell.h"
 #include "ash/system/tray/system_tray.h"
 #include "ash/system/tray/system_tray_delegate.h"
 #include "ash/system/tray/system_tray_item.h"
 #include "ash/system/tray/tray_constants.h"
 #include "ash/wm/window_animations.h"
 #include "base/message_loop.h"
 #include "ui/aura/window.h"
 #include "ui/compositor/layer.h"
 #include "ui/compositor/layer_animation_observer.h"
 #include "ui/compositor/scoped_layer_animation_settings.h"
 #include "ui/gfx/canvas.h"
 #include "ui/views/layout/box_layout.h"
 #include "ui/views/view.h"
+#include "ui/views/widget/widget.h"
 
 namespace ash {
 
 namespace {
 
 const int kAnimationDurationForPopupMS = 200;
 
 // Normally a detailed view is the same size as the default view. However,
 // when showing a detailed view directly (e.g. clicking on a notification),
 // we may not know the height of the default view, or the default view may
@@ skipping 442 matching lines @@
 }
 
 void SystemTrayBubble::OnWidgetClosing(views::Widget* widget) {
   CHECK_EQ(bubble_widget_, widget);
   bubble_widget_ = NULL;
   tray_->RemoveBubble(this);
 }
 
 }  // namespace internal
 }  // namespace ash