[d8 Workers] Fix bug creating Worker during main thread termination

test/mjsunit/regress/regress-crbug-511880.js

Index: test/mjsunit/regress/regress-crbug-511880.js
diff --git a/test/mjsunit/regress/regress-crbug-511880.js b/test/mjsunit/regress/regress-crbug-511880.js
new file mode 100644
index 0000000000000000000000000000000000000000..f9b05ff7bc2f12d775f478a569194606e454b45e
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-511880.js
@@ -0,0 +1,13 @@
+// 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 __v_8 =
+    `var __v_9 = new Worker('postMessage(42)');
+     onmessage = function(parentMsg) {
+       __v_9.postMessage(parentMsg);
+     };`;
+  var __v_9 = new Worker(__v_8);
+  __v_9.postMessage(9);
+}