Fix cluster-fuzz found regression in d8 Workers

test/mjsunit/regress/regress-crbug-505778.js

Index: test/mjsunit/regress/regress-crbug-505778.js
diff --git a/test/mjsunit/regress/regress-crbug-504136.js b/test/mjsunit/regress/regress-crbug-505778.js
similarity index 62%
copy from test/mjsunit/regress/regress-crbug-504136.js
copy to test/mjsunit/regress/regress-crbug-505778.js
index 0061ebd91db014a9d171fcf547d537c7d7a5ef1d..9f14fd08002e6fccd5ee4266c0f63d07fe6926a0 100644
--- a/test/mjsunit/regress/regress-crbug-504136.js
+++ b/test/mjsunit/regress/regress-crbug-505778.js
@@ -3,8 +3,9 @@
 // found in the LICENSE file.
 
 if (this.Worker) {
-  function __f_5() {}
-  var __v_10 = new Worker(__f_5);
-  __v_10.terminate();
- __v_10.getMessage();
+  function __f_3() {
+    onmessage = function() {}
+  }
+  var __v_7 = new Worker(__f_3);
+  __v_7.postMessage("");
 }