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

ash/aura/wm_shelf_aura.h

 // 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.
 
 #ifndef ASH_AURA_WM_SHELF_AURA_H_
 #define ASH_AURA_WM_SHELF_AURA_H_
 
 #include "ash/ash_export.h"
 #include "ash/common/shelf/wm_shelf.h"
 #include "base/macros.h"
 
 namespace ash {
 
 class ShelfBezelEventHandler;
 
 // Aura implementation of WmShelf.
 class ASH_EXPORT WmShelfAura : public WmShelf {
  public:
   WmShelfAura();
   ~WmShelfAura() override;
 
   // WmShelf:
   WmDimmerView* CreateDimmerView(bool disable_animations_for_test) override;
-  void SetShelfLayoutManager(ShelfLayoutManager* manager) override;
+  void CreateShelfWidget(WmWindow* root) override;
   void WillDeleteShelfLayoutManager() override;
   void WillChangeVisibilityState(ShelfVisibilityState new_state) override;
 
  private:
   class AutoHideEventHandler;
 
   // Forwards mouse and gesture events to ShelfLayoutManager for auto-hide.
   // TODO(mash): Facilitate simliar functionality in mash: crbug.com/631216
   std::unique_ptr<AutoHideEventHandler> auto_hide_event_handler_;
 
   // Forwards touch gestures on a bezel sensor to the shelf.
   // TODO(mash): Facilitate simliar functionality in mash: crbug.com/636647
   std::unique_ptr<ShelfBezelEventHandler> bezel_event_handler_;
 
   DISALLOW_COPY_AND_ASSIGN(WmShelfAura);
 };
 
 }  // namespace ash
 
 #endif  // ASH_AURA_WM_SHELF_AURA_H_