Fixed some cctest flakiness on mac.

src/debug.cc

Index: src/debug.cc
diff --git a/src/debug.cc b/src/debug.cc
index 3c2bfa80c6e8e2b792ced29f25db7935472fec38..ec658d68f06844f7ca1a93697b763b7311be58ff 100644
--- a/src/debug.cc
+++ b/src/debug.cc
@@ -2498,6 +2498,11 @@ void Debugger::StopAgent() {
 }
 
 
+void Debugger::WaitForAgent() {
+  if (agent_ != NULL)
+    agent_->WaitUntilListening();
+}
+
 MessageImpl MessageImpl::NewEvent(DebugEvent event,
                                   bool running,
                                   Handle<JSObject> exec_state,