mash: Move WmShelfAura ownership to RootWindowController and init it earlier

ash/shelf/shelf.h

Index: ash/shelf/shelf.h
diff --git a/ash/shelf/shelf.h b/ash/shelf/shelf.h
index bb962d0f5c6ec506cf7305830b0f407745a6e94e..1292e1583c39fdcc7b984ebc13cce2b6b2cd5040 100644
--- a/ash/shelf/shelf.h
+++ b/ash/shelf/shelf.h
@@ -40,7 +40,6 @@ class FocusCycler;
 class ShelfDelegate;
 class ShelfIconObserver;
 class ShelfModel;
-class WmShelfAura;
 
 namespace test {
 class ShelfTestAPI;
@@ -155,12 +154,9 @@ class ASH_EXPORT Shelf {
   // Returns ApplicationDragAndDropHost for this shelf.
   app_list::ApplicationDragAndDropHost* GetDragAndDropHostForAppList();
 
-  WmShelfAura* wm_shelf() { return wm_shelf_.get(); }
-
  private:
   friend class test::ShelfTestAPI;
 
-  std::unique_ptr<WmShelfAura> wm_shelf_;
   ShelfDelegate* delegate_;
   ShelfWidget* shelf_widget_;
   ShelfView* shelf_view_;