[d8] bounds-check before getting Shell::Worker internal field

test/mjsunit/regress/regress-4271.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) {
+  // Throw rather than overflow internal field index
+  assertThrows(function() {
+    Worker.prototype.terminate();
+  });
+
+  assertThrows(function() {
+    Worker.prototype.getMessage();
+  });
+
+  assertThrows(function() {
+    Worker.prototype.postMessage({});
+  });
+
+  // Don't throw for real worker
+  var worker = new Worker('');
+  worker.getMessage();
+  worker.postMessage({});
+  worker.terminate();
+}