mash: Avoid shelf crashes on display removal.

ash/mus/bridge/wm_shelf_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/bridge/wm_shelf_mus.h"
 
 #include "ash/common/shelf/shelf_widget.h"
 #include "ash/common/wm_root_window_controller.h"
 #include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
 
 namespace ash {
 namespace mus {
 
 WmShelfMus::WmShelfMus(WmWindow* root_window) {
   DCHECK(root_window);
   WmShell::Get()->CreateShelfDelegate();
   CreateShelfWidget(root_window);
   InitializeShelf();
   WmShell::Get()->NotifyShelfCreatedForRootWindow(root_window);
   shelf_widget()->PostCreateShelf();
 }
 
 WmShelfMus::~WmShelfMus() {
+  ShutdownShelfWidget();
   DestroyShelfWidget();
   ShutdownShelf();
 }
 
 void WmShelfMus::WillDeleteShelfLayoutManager() {
   ShutdownShelfWidget();
   WmShelf::WillDeleteShelfLayoutManager();
 }
 
 }  // namespace mus
 }  // namespace ash