Fix race in ServiceWorkerBackgroundSyncTest.Sync

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

Index: chrome/test/data/extensions/api_test/service_worker/sync/sw.js
diff --git a/chrome/test/data/extensions/api_test/service_worker/sync/sw.js b/chrome/test/data/extensions/api_test/service_worker/sync/sw.js
index fc0f26a467413c310ec9e2fe5e5cc1f3c57fd4db..3662d57429b85a60086557321b64b0c2eefc0ff8 100644
--- a/chrome/test/data/extensions/api_test/service_worker/sync/sw.js
+++ b/chrome/test/data/extensions/api_test/service_worker/sync/sw.js
@@ -21,4 +21,5 @@ this.onsync = function(e) {
 
 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.

 };