Fix cluster-fuzz regression when getting message from Worker

test/mjsunit/d8-worker.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 110 matching lines @@
   var ab2 = createArrayBuffer(32);
   w.postMessage(ab2, [ab2]);
   assertEquals(0, ab2.byteLength);  // ArrayBuffer should be neutered.
 
   assertEquals("undefined", typeof foo);
 
   // Read a message from the worker.
   assertEquals("DONE", w.getMessage());
 
   w.terminate();
+
+
+  // Make sure that the main thread doesn't block forever in getMessage() if
+  // the worker dies without posting a message.
+  function f2() {}
+  var w2 = new Worker(f2);
+  var msg = w2.getMessage();
+  assertEquals(undefined, msg);
 }