Generalizing TryLock pattern to ThreadListLocker

Change-Id: Icfe05fac35f72e59bc8d6ec5805020fff3c082d2
diff --git a/src/thread_list.h b/src/thread_list.h
index 125a559..3a20405 100644
--- a/src/thread_list.h
+++ b/src/thread_list.h
@@ -77,6 +77,7 @@
 
   friend class Thread;
   friend class ThreadListLock;
+  friend class ThreadListLocker;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadList);
 };