Adds TraySessionLengthLimitTest to ash_unittests.

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
index 2a4f52752678a5d9a4fe0767e45ff50bc5c2dd3a..3fcba64d7e1e2934dde2db5d24cbc86e0a86837b 100644
--- a/ash/system/session_length_limit/session_length_limit_observer.h
+++ b/ash/system/session_length_limit/session_length_limit_observer.h
@@ -5,10 +5,12 @@
 #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() {}