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

ash/common/shelf/wm_shelf.h

Index: ash/common/shelf/wm_shelf.h
diff --git a/ash/common/shelf/wm_shelf.h b/ash/common/shelf/wm_shelf.h
index aa1abe5ff65969e233acd9d26a3b86c74ba31265..514c1487982b0344b95247d67a9a7f8f03a386a2 100644
--- a/ash/common/shelf/wm_shelf.h
+++ b/ash/common/shelf/wm_shelf.h
@@ -39,13 +39,15 @@ class ASH_EXPORT WmShelf : public ShelfLayoutManagerObserver {
   // widget may not exist, or the shelf may not be visible.
   static WmShelf* ForWindow(WmWindow* window);
 
-  // TODO(jamescook): Create the ShelfLayoutManager in this class.
-  virtual void SetShelfLayoutManager(ShelfLayoutManager* manager);
+  virtual void CreateShelfWidget(WmWindow* root);
+  void ShutdownShelfWidget();
+  void DestroyShelfWidget();
+
   ShelfLayoutManager* shelf_layout_manager() const {
     return shelf_layout_manager_;
   }
 
-  ShelfWidget* shelf_widget() { return shelf_widget_; }
+  ShelfWidget* shelf_widget() { return shelf_widget_.get(); }
 
   // Creates the shelf view.
   void InitializeShelf();
@@ -155,8 +157,7 @@ class ASH_EXPORT WmShelf : public ShelfLayoutManagerObserver {
   // ShelfWidget and lifetimes are managed by the container windows themselves.
   ShelfLayoutManager* shelf_layout_manager_ = nullptr;
 
-  // TODO(jamescook): Move ShelfWidget ownership here.
-  ShelfWidget* shelf_widget_ = nullptr;
+  std::unique_ptr<ShelfWidget> shelf_widget_;
 
   // Internal implementation detail. Do not expose externally. Owned by views
   // hierarchy. Null before login and in secondary display init.