chromeos: Convert system tray session length limit to mojo

ash/system/session/session_length_limit_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_SYSTEM_SESSION_SESSION_LENGTH_LIMIT_OBSERVER_H_
-#define ASH_SYSTEM_SESSION_SESSION_LENGTH_LIMIT_OBSERVER_H_
-
-#include "ash/ash_export.h"
-
-namespace ash {
-
-// Observer for the session length limit.
-class ASH_EXPORT SessionLengthLimitObserver {
- public:
-  virtual ~SessionLengthLimitObserver() {}
-
-  // Called when the session start time is updated.
-  virtual void OnSessionStartTimeChanged() = 0;
-
-  // Called when the session length limit is updated.
-  virtual void OnSessionLengthLimitChanged() = 0;
-};
-
-}  // namespace ash
-
-#endif  // ASH_SYSTEM_SESSION_SESSION_LENGTH_LIMIT_OBSERVER_H_