[d8 Workers] Add max worker count, throw an exception if too many.

test/mjsunit/regress/regress-crbug-518748.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+if (this.Worker) {
+  var workersToCreate = 1000;
+  var workers = [];
+  assertThrows(function() {
+    for (var i = 0; i < workersToCreate; i++) {
+      workers.push(new Worker(''));
+    }
+  });
+  print('#workers: ', workers.length);
+}