mash: Migrate some system user files to common.

ash/system/user/user_observer.h

Index: ash/system/user/user_observer.h
diff --git a/ash/system/user/user_observer.h b/ash/system/user/user_observer.h
deleted file mode 100644
index 64c2b9de40f60b5d6c8d0a68e4aa4ba1fd0696f0..0000000000000000000000000000000000000000
--- a/ash/system/user/user_observer.h
+++ /dev/null
@@ -1,25 +0,0 @@
-// 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_
-
-#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_