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

ash/common/shelf/shelf_widget.h

Index: ash/common/shelf/shelf_widget.h
diff --git a/ash/common/shelf/shelf_widget.h b/ash/common/shelf/shelf_widget.h
index 1db458c9cf41e5d034b98bf46f9d2e52d9fef2c3..b02a3c351e656c8ec358d6e46bf23ab50bc35502 100644
--- a/ash/common/shelf/shelf_widget.h
+++ b/ash/common/shelf/shelf_widget.h
@@ -34,11 +34,11 @@ class ASH_EXPORT ShelfWidget : public views::Widget,
                                public ShelfBackgroundAnimatorObserver,
                                public ShelfLayoutManagerObserver {
  public:
-  ShelfWidget(WmWindow* shelf_container,
-              WmWindow* status_container,
-              WmShelf* wm_shelf);
+  ShelfWidget(WmWindow* shelf_container, WmShelf* wm_shelf);
   ~ShelfWidget() override;
 
+  void CreateStatusAreaWidget(WmWindow* status_container);
+
   // Returns if shelf alignment option is enabled, and the user is able to
   // adjust the alignment (guest and supervised mode users cannot for example).
   static bool ShelfAlignmentAllowed();
@@ -130,6 +130,8 @@ class ASH_EXPORT ShelfWidget : public views::Widget,
 
   // Owned by the shelf container's window.
   ShelfLayoutManager* shelf_layout_manager_;
+
+  // Owned by the native widget.
   StatusAreaWidget* status_area_widget_;
 
   // |delegate_view_| is the contents view of this widget and is cleaned up