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

ash/mus/bridge/wm_shelf_mus.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_MUS_BRIDGE_WM_SHELF_MUS_H_
 #define ASH_MUS_BRIDGE_WM_SHELF_MUS_H_
 
 #include "ash/common/shelf/wm_shelf.h"
 #include "base/macros.h"
 
 namespace ash {
 
-class ShelfWidget;
-class WmRootWindowController;
+class WmWindow;
 
 namespace mus {
 
 // WmShelf implementation for mus.
 class WmShelfMus : public WmShelf {
  public:
-  WmShelfMus(WmRootWindowController* root_window_controller);
+  explicit WmShelfMus(WmWindow* root_window);
   ~WmShelfMus() override;
 
   // WmShelf:
   void WillDeleteShelfLayoutManager() override;
 
  private:
-  // The shelf widget for this shelf.
-  std::unique_ptr<ShelfWidget> shelf_widget_;
-
   DISALLOW_COPY_AND_ASSIGN(WmShelfMus);
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_BRIDGE_WM_SHELF_MUS_H_