views: Extract Widget observer into its own header file.

ash/system/tray/tray_bubble_view.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/tray_bubble_view.h"
 
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ash/system/tray/tray_constants.h"
 #include "ash/wm/shelf_layout_manager.h"
 #include "grit/ash_strings.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "third_party/skia/include/core/SkPaint.h"
 #include "third_party/skia/include/core/SkPath.h"
 #include "third_party/skia/include/effects/SkBlurImageFilter.h"
 #include "ui/aura/event.h"
 #include "ui/aura/window.h"
 #include "ui/base/accessibility/accessible_view_state.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/insets.h"
 #include "ui/gfx/screen.h"
 #include "ui/views/bubble/bubble_frame_view.h"
 #include "ui/views/layout/box_layout.h"
+#include "ui/views/widget/widget.h"
 
 namespace ash {
 
 namespace {
 
 const int kShadowThickness = 4;
 const int kBottomLineHeight = 1;
 const int kSystemTrayBubbleHorizontalInset = 1;
 const int kSystemTrayBubbleVerticalInset = 1;
 
@@ skipping 352 matching lines @@
       return;
   }
   // Handle clicking outside the bubble and tray. We don't block the event, so
   // it will also be handled by whatever widget was clicked on.
   OnClickedOutsideView();
 }
 
 
 }  // namespace internal
 }  // namespace ash