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

test/mjsunit/regress/regress-crbug-503968.js

Index: test/mjsunit/regress/regress-crbug-503968.js
diff --git a/test/mjsunit/regress/regress-crbug-503968.js b/test/mjsunit/regress/regress-crbug-503968.js
index c0446da94db586271c3dcc468cb3ff289434c22d..78d1c7b98a753557b8674301607535d9354a1c09 100644
--- a/test/mjsunit/regress/regress-crbug-503968.js
+++ b/test/mjsunit/regress/regress-crbug-503968.js
@@ -8,8 +8,6 @@ if (this.Worker) {
     this.l = [new __f_0, new __f_0];
   }
   __v_6 = new __f_1;
-  function __f_4() {
-  }
-  var __v_9 = new Worker(__f_4);
+  var __v_9 = new Worker('');
   __v_9.postMessage(__v_6);
 }