Change d8 Worker API so it takes a string instead of a function.

test/mjsunit/regress/regress-crbug-503578.js

 // 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.
 
 if (this.Worker) {
-  function __f_1() {
-    onmessage = function() {}
-  }
   function __f_0(byteLength) {
     var __v_1 = new ArrayBuffer(byteLength);
     var __v_5 = new Uint32Array(__v_1);
     return __v_5;
   }
-  var __v_6 = new Worker(__f_1);
+  var __v_6 = new Worker('onmessage = function() {}');
   var __v_3 = __f_0(16);
   __v_6.postMessage(__v_3);
 }