Implement monitors.

Change-Id: Ifc7a801f9cbcdfbc1e1af5c905261dfadaa60f45
diff --git a/src/signal_catcher.h b/src/signal_catcher.h
index 16cf9e4..123b38f 100644
--- a/src/signal_catcher.h
+++ b/src/signal_catcher.h
@@ -45,7 +45,7 @@
 
   mutable Mutex lock_;
   bool halt_;
-  pthread_cond_t cond_;
+  ConditionVariable cond_;
   pthread_t pthread_;
   Thread* thread_;
 };