Fix race in ServiceWorkerBackgroundSyncTest.Sync

chrome/test/data/extensions/api_test/service_worker/sync/page.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 registerSyncOnServiceWorker = new Promise(function(resolve, reject) {
   var serviceWorker;
   navigator.serviceWorker.register('sw.js').then(function() {
     // Wait until the service worker is active.
     return navigator.serviceWorker.ready;
   }).then(function(registration) {
     serviceWorker = registration.active;
     return registration.sync.register('send-chats');
   }).then(function() {
     resolve(serviceWorker);
   }).catch(function(err) {
     reject(err);
   });
 });
 
 window.runServiceWorker = function() {
   registerSyncOnServiceWorker.then(function(serviceWorker) {
     var mc = new MessageChannel();
     // Called when ServiceWorker.onsync fires.
     mc.port1.onmessage = function(e) {
+      if (e.data == 'connected') {
+        window.domAutomationController.send('SERVICE_WORKER_READY');
+        return;
+      }
       if (e.data != 'SYNC: send-chats') {
         console.log('SW returned incorrect data: ' + e.data);
         chrome.test.sendMessage('FAIL');  // Fails the test fast.
         return;
       }
       chrome.test.sendMessage(e.data);
     };
     serviceWorker.postMessage('connect', [mc.port2]);
-    window.domAutomationController.send('SERVICE_WORKER_READY');
   }).catch(function(err) {
     window.domAutomationController.send(err);
   });
 };