mash: Move WmShelfAura ownership to RootWindowController and init it earlier

ash/shelf/shelf_widget.h

Index: ash/shelf/shelf_widget.h
diff --git a/ash/shelf/shelf_widget.h b/ash/shelf/shelf_widget.h
index 95b72cead717eb2c2ae1ea551304ccadb716c461..8180c162acd11bb752eeca0e65c4d36bfbf31286 100644
--- a/ash/shelf/shelf_widget.h
+++ b/ash/shelf/shelf_widget.h
@@ -20,6 +20,7 @@ class FocusCycler;
 class Shelf;
 class ShelfLayoutManager;
 class StatusAreaWidget;
+class WmShelfAura;
 class WmWindow;
 class WorkspaceController;
 
@@ -57,7 +58,7 @@ class ASH_EXPORT ShelfWidget : public views::Widget,
   Shelf* shelf() const { return shelf_.get(); }
   StatusAreaWidget* status_area_widget() const { return status_area_widget_; }
 
-  void CreateShelf();
+  void CreateShelf(WmShelfAura* wm_shelf_aura);
   void PostCreateShelf();
 
   // Set visibility of the shelf.