reland of https://codereview.chromium.org/11364053/ w/o removing the per-rootwindow stacking client

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..8da3ca3e0c93abe6d83a769b862a3837a1cdfd3d 100644
--- a/ui/aura/client/stacking_client.h
+++ b/ui/aura/client/stacking_client.h
@@ -21,10 +21,15 @@ 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.
@@ -40,6 +45,7 @@ AURA_EXPORT void SetStackingClient(Window* window,
                                    StackingClient* stacking_client);
 AURA_EXPORT StackingClient* GetStackingClient(Window* window);
 
+
 }  // namespace client
 }  // namespace aura