Second attempt at moving the StackingClient to a property on the RootWindow.

ui/aura/client/stacking_client.h

Index: ui/aura/client/stacking_client.h
===================================================================
--- ui/aura/client/stacking_client.h	(revision 114527)
+++ ui/aura/client/stacking_client.h	(working copy)
@@ -9,20 +9,23 @@
 #include "ui/aura/aura_export.h"
 
 namespace aura {
-
 class Window;
+namespace client {
 
 // An interface implemented by an object that stacks windows.
 class AURA_EXPORT StackingClient {
  public:
   virtual ~StackingClient() {}
 
-  // Called by the Window when its parent is set to NULL. The delegate is given
-  // an opportunity to inspect the window and add it to a default parent window
-  // of its choosing.
-  virtual void AddChildToDefaultParent(Window* window) = 0;
+  // Called by the Window when its parent is set to NULL, returns the window
+  // that |window| should be added to instead.
+  virtual Window* GetDefaultParent(Window* window) = 0;
 };
 
+AURA_EXPORT void SetStackingClient(StackingClient* stacking_client);
+AURA_EXPORT StackingClient* GetStackingClient();
+
+}  // namespace client
 }  // namespace aura
 
 #endif  // UI_AURA_CLIENT_STACKING_CLIENT_H_