Fix race in ServiceWorkerBackgroundSyncTest.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];
+  port.postMessage('connected');

[iclelland, 2016/03/16 15:46:33]

Should this check to see if e.data is "connect"? I don't know if there's ever
any *other* reason to postMessage to the service worker, but if we ever do, this
will echo back every time, and the page will end up triggering
SERVICE_WORKER_READY every time.

[jkarlin, 2016/03/16 16:59:37]

Sure. Done.

 };