mash: Move WmShelfAura ownership to RootWindowController and init it earlier

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 "ash/shelf/shelf_icon_observer.h"
 #include "ash/shelf/shelf_layout_manager_observer.h"
 #include "base/macros.h"
 #include "base/observer_list.h"
 
 namespace ash {
 
 class Shelf;
 class ShelfLayoutManager;
 
 // Aura implementation of WmShelf.
 class ASH_EXPORT WmShelfAura : public WmShelf,
                                public ShelfLayoutManagerObserver,
                                public ShelfIconObserver {
  public:
-  explicit WmShelfAura(Shelf* shelf);
+  WmShelfAura();
   ~WmShelfAura() override;
 
+  // This object is initialized in multiple steps as the RootWindowController
+  // constructs the components of the shelf.
+  void SetShelfLayoutManager(ShelfLayoutManager* shelf_layout_manager);
+  void SetShelf(Shelf* shelf);
+
+  void Shutdown();
+
   static Shelf* GetShelf(WmShelf* shelf);
 
  private:
+  void ResetShelfLayoutManager();
+
   // WmShelf:
   WmWindow* GetWindow() override;
   ShelfAlignment GetAlignment() const override;
   ShelfBackgroundType GetBackgroundType() const override;
   void UpdateVisibilityState() override;
   ShelfVisibilityState GetVisibilityState() const override;
   void UpdateIconPositionForWindow(WmWindow* window) override;
   gfx::Rect GetScreenBoundsOfItemIconForWindow(WmWindow* window) override;
   void AddObserver(WmShelfObserver* observer) override;
   void RemoveObserver(WmShelfObserver* observer) override;
 
   // ShelfLayoutManagerObserver:
   void WillDeleteShelfLayoutManager() override;
   void OnBackgroundUpdated(ShelfBackgroundType background_type,
                            BackgroundAnimatorChangeType change_type) override;
   void WillChangeVisibilityState(ShelfVisibilityState new_state) override;
 
   // ShelfIconObserver:
   void OnShelfIconPositionsChanged() override;
 
-  Shelf* shelf_;
+  // May be null during login.
+  Shelf* shelf_ = nullptr;
+
+  // Cached separately because it may be destroyed before |shelf_|.
+  ShelfLayoutManager* shelf_layout_manager_ = nullptr;
+
   base::ObserverList<WmShelfObserver> observers_;
-  // ShelfLayoutManager is cached separately as it may be destroyed before
-  // WmShelfAura.
-  ShelfLayoutManager* shelf_layout_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(WmShelfAura);
 };
 
 }  // namespace ash
 
 #endif  // ASH_AURA_WM_SHELF_AURA_H_