Extension SW - add test for ServiceWorkerRegistration.sync.

chrome/test/data/extensions/api_test/service_worker/sync/sw.js

+// Copyright 2015 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.
+
+var port = null;
+this.onsync = function(e) {
+  if (port) {
+    e.waitUntil(new Promise(function(resolve) {
+      // Add a small delay to respond so we can exercise and test the
+      // offline->online transition.
+      // NOTE: the following setTimeout() is not a requirement for this test. It
+      // is here to just demonstrate that the test passes with a bit of
+      // asynchrony.
+      setTimeout(function() {
+        port.postMessage('SYNC: ' + e.tag);
+        resolve();
+      }, 0);
+    }));
+  }
+};
+
+this.onmessage = function(e) {
+  port = e.ports[0];
+};