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

test/mjsunit/regress/regress-crbug-504729.js

Index: test/mjsunit/regress/regress-crbug-504729.js
diff --git a/test/mjsunit/regress/regress-crbug-504729.js b/test/mjsunit/regress/regress-crbug-504729.js
index 2a790542013bc9f7240982cb576b45e4d5a10afd..435cafee877a60e23bf6e92123ba6ac153fc2589 100644
--- a/test/mjsunit/regress/regress-crbug-504729.js
+++ b/test/mjsunit/regress/regress-crbug-504729.js
@@ -5,5 +5,5 @@
 if (this.Worker) {
   Function.prototype.toString = "foo";
   function __f_7() {}
-  assertThrows(function() { var __v_5 = new Worker(__f_7); });
+  assertThrows(function() { var __v_5 = new Worker(__f_7.toString()); });
 }