Adds TraySessionLengthLimitTest to ash_unittests.

ash/system/session_length_limit/session_length_limit_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_SESSION_LENGTH_LIMIT_SESSION_LENGTH_LIMIT_OBSERVER_H_
 #define ASH_SYSTEM_SESSION_LENGTH_LIMIT_SESSION_LENGTH_LIMIT_OBSERVER_H_
 
+#include "ash/ash_export.h"
+
 namespace ash {
 
 // Observer for the session length limit.
-class SessionLengthLimitObserver {
+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_LENGTH_LIMIT_SESSION_LENGTH_LIMIT_OBSERVER_H_