Fix cluster-fuzz regression with Workers when serializing empty string

test/mjsunit/regress/regress-crbug-503991.js

Index: test/mjsunit/regress/regress-crbug-503991.js
diff --git a/test/message/super-in-function.js b/test/mjsunit/regress/regress-crbug-503991.js
similarity index 59%
copy from test/message/super-in-function.js
copy to test/mjsunit/regress/regress-crbug-503991.js
index edaa0e4eadc7befdb7ed28a3e06eaa827c4388f0..aa0a28a342127ad4d61ba3dfd592dd7779a9eeb3 100644
--- a/test/message/super-in-function.js
+++ b/test/mjsunit/regress/regress-crbug-503991.js
@@ -1,10 +1,10 @@
 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
-//
-// Flags: --harmony-classes
-'use strict';
 
-function f() {
-  super.x();
+if (this.Worker) {
+  __v_3 = "";
+  function __f_1() {}
+  var __v_6 = new Worker(__f_1);
+  __v_6.postMessage(__v_3);
 }