ash: Move ShelfWidget ownership to WmShelf and refactor access to it

ash/mus/accelerators/accelerator_controller_delegate_mus.cc

 // Copyright 2016 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/mus/accelerators/accelerator_controller_delegate_mus.h"
 
 #include "base/logging.h"
 
 namespace ash {
 namespace mus {
@@ skipping 15 matching lines @@
     case DEBUG_TOGGLE_SHOW_FPS_COUNTER:
     case DEBUG_TOGGLE_SHOW_PAINT_RECTS:
     case DEBUG_TOGGLE_WALLPAPER_MODE:
     case MAGNIFY_SCREEN_ZOOM_IN:
     case MAGNIFY_SCREEN_ZOOM_OUT:
     case ROTATE_SCREEN:
     case ROTATE_WINDOW:
     case SCALE_UI_DOWN:
     case SCALE_UI_RESET:
     case SCALE_UI_UP:
-    case SHOW_MESSAGE_CENTER_BUBBLE:
     case SHOW_SYSTEM_TRAY_BUBBLE:
     case TAKE_PARTIAL_SCREENSHOT:
     case TAKE_SCREENSHOT:
     case TAKE_WINDOW_SCREENSHOT:
     case UNPIN:
       NOTIMPLEMENTED();
       return false;
 
 #if defined(OS_CHROMEOS)
     case DEBUG_ADD_REMOVE_DISPLAY:
@@ skipping 36 matching lines @@
     const char* const notification_id,
     int message_id,
     int old_shortcut_id,
     int new_shortcut_id) {
   // TODO: http://crbug.com/630316.
   NOTIMPLEMENTED();
 }
 
 }  // namespace mus
 }  // namespace ash