mash: remove more shell/shelf WmWindow usage.

ash/scoped_root_window_for_new_windows.h

 // Copyright 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 ASH_SCOPED_ROOT_WINDOW_FOR_NEW_WINDOWS_H_
 #define ASH_SCOPED_ROOT_WINDOW_FOR_NEW_WINDOWS_H_
 
 #include "ash/ash_export.h"
 #include "base/macros.h"
 
+namespace aura {
+class Window;
+}
+
 namespace ash {
 
-class WmWindow;
-
 // Constructing a ScopedRootWindowForNewWindows allows temporarily
 // switching a target root window so that a new window gets created
 // in the same window where a user interaction happened.
 // An example usage is to specify the target root window when creating
 // a new window using launcher's icon.
 class ASH_EXPORT ScopedRootWindowForNewWindows {
  public:
-  explicit ScopedRootWindowForNewWindows(WmWindow* new_root);
+  explicit ScopedRootWindowForNewWindows(aura::Window* new_root);
   ~ScopedRootWindowForNewWindows();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ScopedRootWindowForNewWindows);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SCOPED_ROOT_WINDOW_FOR_NEW_WINDOWS_H_