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

ash/shell.h

 // 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.
 
 #ifndef ASH_SHELL_H_
 #define ASH_SHELL_H_
 
 #include <memory>
 #include <utility>
 #include <vector>
@@ skipping 441 matching lines @@
   ~Shell() override;
 
   void Init(const ShellInitParams& init_params);
 
   // Initializes virtual keyboard controller.
   void InitKeyboard();
 
   // Initializes the root window so that it can host browser windows.
   void InitRootWindow(aura::Window* root_window);
 
-  // Prepares the shelf to be deleted.
-  void ShutdownShelf();
-
   // SystemModalContainerEventFilterDelegate:
   bool CanWindowReceiveEvents(aura::Window* window) override;
 
   // Overridden from ui::EventTarget:
   bool CanAcceptEvent(const ui::Event& event) override;
   EventTarget* GetParentTarget() override;
   std::unique_ptr<ui::EventTargetIterator> GetChildIterator() const override;
   ui::EventTargeter* GetEventTargeter() override;
 
   static Shell* instance_;
@@ skipping 118 matching lines @@
   base::SequencedWorkerPool* blocking_pool_;
 
   bool in_mus_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(Shell);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SHELL_H_