Make GetRootWindow() return a Window instead of a RootWindow.

ash/shell/context_menu.h

Index: ash/shell/context_menu.h
diff --git a/ash/shell/context_menu.h b/ash/shell/context_menu.h
index a07310ece953574e2e369b9feca5ac01cd9fd6eb..32421ca5608574d14dd3f37b65d583bed41988f8 100644
--- a/ash/shell/context_menu.h
+++ b/ash/shell/context_menu.h
@@ -21,7 +21,7 @@ namespace shell {
 class ContextMenu : public ui::SimpleMenuModel,
                     public ui::SimpleMenuModel::Delegate {
  public:
-  explicit ContextMenu(aura::RootWindow* root);
+  explicit ContextMenu(aura::Window* root);
   virtual ~ContextMenu();
 
   // ui::SimpleMenuModel::Delegate overrides:
@@ -38,7 +38,7 @@ class ContextMenu : public ui::SimpleMenuModel,
     MENU_ALIGNMENT_MENU,
   };
 
-  aura::RootWindow* root_window_;
+  aura::Window* root_window_;
 
   ShelfAlignmentMenu alignment_menu_;