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

test/mjsunit/regress/regress-crbug-503698.js

Index: test/mjsunit/regress/regress-crbug-503698.js
diff --git a/test/mjsunit/regress/regress-crbug-503698.js b/test/mjsunit/regress/regress-crbug-503698.js
index 274eb5a99e9bc392107d933400f1ff2a3aab0e13..415d1bc81be5ac46fb358ce8ec681b1e5c276e07 100644
--- a/test/mjsunit/regress/regress-crbug-503698.js
+++ b/test/mjsunit/regress/regress-crbug-503698.js
@@ -5,6 +5,5 @@
 // Flags: --invoke-weak-callbacks
 
 if (this.Worker) {
-  function __f_1() {}
-  var __v_6 = new Worker(__f_1);
+  var __v_6 = new Worker('');
 }