Fixes bug in handling restacking because of transients

ui/aura/client/transient_window_client_observer.h

 // Copyright 2016 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_TRANSIENT_WINDOW_CLIENT_OBSERVER_H_
 #define UI_AURA_CLIENT_TRANSIENT_WINDOW_CLIENT_OBSERVER_H_
 
 #include "ui/aura/aura_export.h"
 
 namespace aura {
 
 class Window;
 
 namespace client {
 
 class AURA_EXPORT TransientWindowClientObserver {
  public:
   // Called when a window is added as a transient child. This is called once
   // the child is added as a transient, but before any restacking occurs.
   virtual void OnTransientChildWindowAdded(Window* parent,
                                            Window* transient_child) = 0;
 
   // Called when a window is removed as a transient child. This is called once
   // the child is removed as a transient, but before any restacking occurs.
   virtual void OnTransientChildWindowRemoved(Window* parent,
                                              Window* transient_child) = 0;
 
+  // Called when adding or removing a transient results in restacking. |window|
+  // is the window being stacked above |source|. |source| is the window the
+  // transient was added or removed from. This is followed by a call to
+  // Window::StackChildAbove() and then OnDidRestackTransientChildAbove().
+  virtual void OnWillRestackTransientChildAbove(Window* source,

[msw, 2016/11/21 19:56:33]

nit: are there better names we can use here? like |parent| and |transient_child|
above?

[sky, 2016/11/21 21:31:43]

Done.

+                                                Window* window) = 0;
+  virtual void OnDidRestackTransientChildAbove(Window* source,
+                                               Window* window) = 0;
+
  protected:
   virtual ~TransientWindowClientObserver() {}
 };
 
 }  // namespace client
 }  // namespace aura
 
 #endif  // UI_AURA_CLIENT_TRANSIENT_WINDOW_CLIENT_OBSERVER_H_