make StackingClient dispatch to either ash or desktop

ash/wm/stacking_controller.cc

Index: ash/wm/stacking_controller.cc
diff --git a/ash/wm/stacking_controller.cc b/ash/wm/stacking_controller.cc
index f5b2c065c56639c0af89051b21fc74531893ec1a..b12cdac0421dbe7553fe0e7b3955ae2ab48ad5d0 100644
--- a/ash/wm/stacking_controller.cc
+++ b/ash/wm/stacking_controller.cc
@@ -56,7 +56,6 @@ bool IsWindowModal(aura::Window* window) {
 // StackingController, public:
 
 StackingController::StackingController() {
-  aura::client::SetStackingClient(this);
 }
 
 StackingController::~StackingController() {
@@ -65,7 +64,8 @@ StackingController::~StackingController() {
 ////////////////////////////////////////////////////////////////////////////////
 // StackingController, aura::StackingClient implementation:
 
-aura::Window* StackingController::GetDefaultParent(aura::Window* window,
+aura::Window* StackingController::GetDefaultParent(aura::Window* context,
+                                                   aura::Window* window,
                                                    const gfx::Rect& bounds) {
   aura::RootWindow* target_root = NULL;
   if (window->transient_parent()) {