make StackingClient dispatch to either ash or desktop

ui/views/widget/widget.h

Index: ui/views/widget/widget.h
diff --git a/ui/views/widget/widget.h b/ui/views/widget/widget.h
index cd4d0ce52e3a215c5c9719b46e4ba09fe78a5640..63f4967958733c672a839631c0fea1b0839edb6d 100644
--- a/ui/views/widget/widget.h
+++ b/ui/views/widget/widget.h
@@ -191,6 +191,10 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
     // Only used by NativeWidgetAura. Specifies the type of layer for the
     // aura::Window. Default is LAYER_TEXTURED.
     ui::LayerType layer_type;
+    // Only used by Aura. Provides additional context (generally a RootWindow)
+    // during creation to allow the widget to determine which desktop type it
+    // will belong to. NULL indicates a normal native desktop.
+    gfx::NativeView context;
   };
 
   Widget();