Prevent a service worker from keeping itself alive by self postMessage.

content/test/data/service_worker/worker_message_to_self.js

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+this.onactivate = function(event) {
+    self.registration.active.postMessage('foo');
+};
+
+this.onmessage = function(event) {
+    console.log("MESSAGE");
+    self.registration.active.postMessage('foo');
+};