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

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 74d0f125ee2250c829762f8ae26f5a41b4ec6317..fbd85b8e6e6fa7b71c4d3704e171b2b06980697a 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -463,9 +463,11 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
       aura::Window* root_window) const;
 
   // Sets/gets shelf's alignment on |root_window|.
-  void SetShelfAlignment(ShelfAlignment alignment,
-                         aura::Window* root_window);
-  ShelfAlignment GetShelfAlignment(const aura::Window* root_window);
+  void SetShelfAlignment(ShelfAlignment alignment, aura::Window* root_window);
+  ShelfAlignment GetShelfAlignment(const aura::Window* root_window) const;
+
+  // Called when the alignment for a shelf changes.
+  void OnShelfAlignmentChanged(aura::Window* root_window);
 
   // Notifies |observers_| when entering or exiting fullscreen mode in
   // |root_window|.