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

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

Index: test/mjsunit/regress/regress-crbug-503991.js
diff --git a/test/mjsunit/regress/regress-crbug-503991.js b/test/mjsunit/regress/regress-crbug-503991.js
index aa0a28a342127ad4d61ba3dfd592dd7779a9eeb3..6a3b0de7597013f6817dfe6f496670bd469dc93a 100644
--- a/test/mjsunit/regress/regress-crbug-503991.js
+++ b/test/mjsunit/regress/regress-crbug-503991.js
@@ -4,7 +4,6 @@
 
 if (this.Worker) {
   __v_3 = "";
-  function __f_1() {}
-  var __v_6 = new Worker(__f_1);
+  var __v_6 = new Worker('');
   __v_6.postMessage(__v_3);
 }