mash: Migrate tray date view to //ash/common types

ash/system/date/clock_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_DATE_CLOCK_OBSERVER_H_
-#define ASH_SYSTEM_DATE_CLOCK_OBSERVER_H_
-
-#include "ash/ash_export.h"
-
-namespace ash {
-
-class ASH_EXPORT ClockObserver {
- public:
-  virtual ~ClockObserver() {}
-
-  virtual void OnDateFormatChanged() = 0;
-  virtual void OnSystemClockTimeUpdated() = 0;
-  virtual void OnSystemClockCanSetTimeChanged(bool can_set_time) = 0;
-
-  // Force a refresh (e.g. after the system is resumed).
-  virtual void Refresh() = 0;
-};
-
-}  // namespace ash
-
-#endif  // ASH_SYSTEM_DATE_CLOCK_OBSERVER_H_