views: Extract Widget observer into its own header file.

ash/launcher/overflow_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/launcher/overflow_bubble.h"
 
 #include <algorithm>
 
 #include "ash/launcher/launcher_types.h"
 #include "ash/launcher/launcher_view.h"
 #include "ui/gfx/insets.h"
 #include "ui/gfx/screen.h"
 #include "ui/views/bubble/bubble_delegate.h"
 #include "ui/views/bubble/bubble_frame_view.h"
+#include "ui/views/widget/widget.h"
 
 namespace ash {
 namespace internal {
 
 namespace {
 
 // This should be the same color as the darkest launcher bar.
 const SkColor kLauncherColor = SkColorSetARGB(0x80, 0, 0, 0);
 
 // Max bubble size to screen size ratio.
@@ skipping 258 matching lines @@
   bubble_ = NULL;
 }
 
 void OverflowBubble::OnWidgetClosing(views::Widget* widget) {
   DCHECK(widget == bubble_->GetWidget());
   bubble_ = NULL;
 }
 
 }  // namespace internal
 }  // namespace ash