Revert "chromeos: Move files in //ash/common to //ash, part 1"

ash/common/system/user/user_observer.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 ASH_SYSTEM_USER_USER_OBSERVER_H_
-#define ASH_SYSTEM_USER_USER_OBSERVER_H_
+#ifndef ASH_COMMON_SYSTEM_USER_USER_OBSERVER_H_
+#define ASH_COMMON_SYSTEM_USER_USER_OBSERVER_H_
 
 #include "ash/ash_export.h"
 
 namespace ash {
 
 class ASH_EXPORT UserObserver {
  public:
   virtual ~UserObserver() {}
 
   // A user got updated / changed.
   virtual void OnUserUpdate() = 0;
 
   // A user was added to the existing session.
   virtual void OnUserAddedToSession() = 0;
 };
 
 }  // namespace ash
 
-#endif  // ASH_SYSTEM_USER_USER_OBSERVER_H_
+#endif  // ASH_COMMON_SYSTEM_USER_USER_OBSERVER_H_