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

ui/aura/client/stacking_client.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_AURA_CLIENT_STACKING_CLIENT_H_
 #define UI_AURA_CLIENT_STACKING_CLIENT_H_
 
 #include "ui/aura/aura_export.h"
 
 namespace gfx {
 class Rect;
 }
 
 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, 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
 
 #endif  // UI_AURA_CLIENT_STACKING_CLIENT_H_