mash: Convert system tray logout button to wm common types

ash/common/system/chromeos/session/last_window_closed_observer.h

 // Copyright 2014 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_COMMON_SYSTEM_CHROMEOS_SESSION_LAST_WINDOW_CLOSED_OBSERVER_H_
 #define ASH_COMMON_SYSTEM_CHROMEOS_SESSION_LAST_WINDOW_CLOSED_OBSERVER_H_
 
+#include "ash/ash_export.h"
+
 namespace ash {
 
-class LastWindowClosedObserver {
+class ASH_EXPORT LastWindowClosedObserver {
  public:
   virtual void OnLastWindowClosed() = 0;
 
  protected:
   virtual ~LastWindowClosedObserver() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SYSTEM_CHROMEOS_SESSION_LAST_WINDOW_CLOSED_OBSERVER_H_