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

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

Index: test/mjsunit/regress/regress-crbug-503578.js
diff --git a/test/mjsunit/regress/regress-crbug-503578.js b/test/mjsunit/regress/regress-crbug-503578.js
index c32f8da06d7c5cec82e13ba93d5d3c0785525425..1274d91ffec5030610d583acd9b7b22726b87cf2 100644
--- a/test/mjsunit/regress/regress-crbug-503578.js
+++ b/test/mjsunit/regress/regress-crbug-503578.js
@@ -3,15 +3,12 @@
 // 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);
 }