Pass shelf instances, not root windows, for ash context menus.

ash/shelf/shelf_alignment_menu.h

Index: ash/shelf/shelf_alignment_menu.h
diff --git a/ash/shelf/shelf_alignment_menu.h b/ash/shelf/shelf_alignment_menu.h
index add20f089de9fcaee281fb6c47f9bf23fd4d7b24..98d1eeff2546fe2d3cca76c8954a84826985ad34 100644
--- a/ash/shelf/shelf_alignment_menu.h
+++ b/ash/shelf/shelf_alignment_menu.h
@@ -9,17 +9,15 @@
 #include "base/macros.h"
 #include "ui/base/models/simple_menu_model.h"
 
-namespace aura {
-class RootWindow;
-}
-
 namespace ash {
 
+class Shelf;
+
 // Submenu for choosing the alignment of the launcher.
 class ASH_EXPORT ShelfAlignmentMenu : public ui::SimpleMenuModel,
                                       public ui::SimpleMenuModel::Delegate {
  public:
-  explicit ShelfAlignmentMenu(aura::Window* root);
+  explicit ShelfAlignmentMenu(Shelf* shelf);
   ~ShelfAlignmentMenu() override;
 
   // ui::SimpleMenuModel::Delegate overrides:
@@ -37,7 +35,7 @@ class ASH_EXPORT ShelfAlignmentMenu : public ui::SimpleMenuModel,
     MENU_ALIGN_BOTTOM,
   };
 
-  aura::Window* root_window_;
+  Shelf* shelf_;
 
   DISALLOW_COPY_AND_ASSIGN(ShelfAlignmentMenu);
 };