Add countdown when session time is limited

ash/system/session_length_limit/session_length_limit_observer.h

Index: ash/system/session_length_limit/session_length_limit_observer.h
diff --git a/ash/system/session_length_limit/session_length_limit_observer.h b/ash/system/session_length_limit/session_length_limit_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..52be2c4e9f91fb87b4aa01e0700928fcf3dd3f83
--- /dev/null
+++ b/ash/system/session_length_limit/session_length_limit_observer.h
@@ -0,0 +1,27 @@
+// 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_SESSION_LENGTH_LIMIT_SESSION_LENGTH_LIMIT_OBSERVER_H_
+#define ASH_SYSTEM_SESSION_LENGTH_LIMIT_SESSION_LENGTH_LIMIT_OBSERVER_H_
+
+#include "base/time.h"
+
+namespace ash {
+
+// Observer for the session length limit.
+class SessionLengthLimitObserver {
+ public:
+  virtual ~SessionLengthLimitObserver() {}
+
+  // Called when the session start time is updated.
+  virtual void OnSessionStartTimeChanged(
+      const base::Time& session_start_time) = 0;
+
+  // Called when the session length limit is updated.
+  virtual void OnSessionLengthLimitChanged(const base::TimeDelta& limit) = 0;
+};
+
+}  // namespace ash
+
+#endif  // ASH_SYSTEM_SESSION_LENGTH_LIMIT_SESSION_LENGTH_LIMIT_OBSERVER_H_