make StackingClient dispatch to either ash or desktop

ui/views/widget/native_widget_aura.cc

Index: ui/views/widget/native_widget_aura.cc
diff --git a/ui/views/widget/native_widget_aura.cc b/ui/views/widget/native_widget_aura.cc
index 807c185d6d73cff299e7602f91907e00b36cf032..bb2cf65dfc9d7cdd45661649cb10eeeb216ecb6a 100644
--- a/ui/views/widget/native_widget_aura.cc
+++ b/ui/views/widget/native_widget_aura.cc
@@ -122,8 +122,8 @@ void NativeWidgetAura::InitNativeWidget(const Widget::InitParams& params) {
     // If the parent is not specified, find the default parent for
     // the |window_| using the desired |window_bounds|.
     if (!parent) {
-      parent = aura::client::GetStackingClient(params.GetParent())->
-          GetDefaultParent(window_, window_bounds);
+      parent = aura::client::GetStackingClient()->
+          GetDefaultParent(params.context, window_, window_bounds);
     } else if (window_bounds == gfx::Rect()) {
       // If a parent is specified but no bounds are given,
       // use the origin of the parent's display so that the widget