make StackingClient dispatch to either ash or desktop

ui/aura/client/stacking_client.h

Index: ui/aura/client/stacking_client.h
diff --git a/ui/aura/client/stacking_client.h b/ui/aura/client/stacking_client.h
index 671c75535b20ce432e2c6df727e99d024b60d9e0..ab75dc9bdc07863836374f7a5def2b8812d2c7a7 100644
--- a/ui/aura/client/stacking_client.h
+++ b/ui/aura/client/stacking_client.h
@@ -21,25 +21,21 @@ class AURA_EXPORT StackingClient {
   virtual ~StackingClient() {}
 
   // Called by the Window when its parent is set to NULL, returns the window
-  // that |window| should be added to instead.
+  // that |window| should be added to instead. |context| provides a Window
+  // (generally a RootWindow) that can be used to determine which desktop type
+  // the default parent should be chosen from.
   // NOTE: this may have side effects. It should only be used when |window| is
   // going to be immediately added.
-  virtual Window* GetDefaultParent(Window* window, const gfx::Rect& bounds) = 0;
+  virtual Window* GetDefaultParent(
+      Window* context,
+      Window* window,
+      const gfx::Rect& bounds) = 0;
 };
 
 // Set/Get the default stacking client.
 AURA_EXPORT void SetStackingClient(StackingClient* stacking_client);
 AURA_EXPORT StackingClient* GetStackingClient();
 
-// Set/Get a stacking client for a specific window. Setting the stacking client
-// sets the stacking client on the window's RootWindow, not the window itself.
-// Likewise getting obtains it from the window's RootWindow. If |window| is
-// non-NULL it must be in a RootWindow. If |window| is NULL, the default
-// stacking client is used.
-AURA_EXPORT void SetStackingClient(Window* window,
-                                   StackingClient* stacking_client);
-AURA_EXPORT StackingClient* GetStackingClient(Window* window);
-
 }  // namespace client
 }  // namespace aura