mash: Migrate locale observer and notification to common.

ash/common/system/locale/locale_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_LOCALE_LOCALE_OBSERVER_H_
-#define ASH_SYSTEM_LOCALE_LOCALE_OBSERVER_H_
+#ifndef ASH_COMMON_SYSTEM_LOCALE_LOCALE_OBSERVER_H_
+#define ASH_COMMON_SYSTEM_LOCALE_LOCALE_OBSERVER_H_
+
+#include <string>
 
 #include "ash/ash_export.h"
 
 namespace ash {
 
 class ASH_EXPORT LocaleObserver {
  public:
   class Delegate {
    public:
     virtual ~Delegate() {}
 
     virtual void AcceptLocaleChange() = 0;
     virtual void RevertLocaleChange() = 0;
   };
 
   virtual ~LocaleObserver() {}
 
   virtual void OnLocaleChanged(Delegate* delegate,
                                const std::string& cur_locale,
                                const std::string& from_locale,
                                const std::string& to_locale) = 0;
 };
 
 }  // namespace ash
 
-#endif  // ASH_SYSTEM_LOCALE_LOCALE_OBSERVER_H_
+#endif  // ASH_COMMON_SYSTEM_LOCALE_LOCALE_OBSERVER_H_