Fixed some cctest flakiness on mac.

src/debug.h

Index: src/debug.h
diff --git a/src/debug.h b/src/debug.h
index d6b2c088d3324a216596fae2731129df7d1f1bf6..d5efc4385e785cfc84a761c2e177115f594dbba3 100644
--- a/src/debug.h
+++ b/src/debug.h
@@ -645,6 +645,9 @@ class Debugger {
   // Stop the debugger agent.
   static void StopAgent();
 
+  // Blocks until the agent has started listening for connections
+  static void WaitForAgent();
+
   // Unload the debugger if possible. Only called when no debugger is currently
   // active.
   static void UnloadDebugger();