Moves TransientWindowClient to ui/aura/client and adds observer

ui/wm/core/wm_state.h

 // Copyright (c) 2013 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_WM_CORE_WM_STATE_H_
 #define UI_WM_CORE_WM_STATE_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "ui/wm/wm_export.h"
 
 namespace wm {
 
 class TransientWindowController;
 class TransientWindowStackingClient;
 
 // Installs state needed by the window manager.
 class WM_EXPORT WMState {
  public:
   WMState();
   ~WMState();
 
-  // WindowStackingClient:
  private:
   std::unique_ptr<TransientWindowStackingClient> window_stacking_client_;
   std::unique_ptr<TransientWindowController> transient_window_client_;
 
   DISALLOW_COPY_AND_ASSIGN(WMState);
 };
 
 }  // namespace wm
 
 #endif  // UI_WM_CORE_WM_STATE_H_