Service Worker: Make ServiceWorkerRegistration.update() return a promise. (Blink Layout 3/3)

LayoutTests/http/tests/serviceworker/update.html

Index: LayoutTests/http/tests/serviceworker/update.html
diff --git a/LayoutTests/http/tests/serviceworker/update.html b/LayoutTests/http/tests/serviceworker/update.html
index 9eb392a9898336b51f14b1d476611b9d50b2046c..5cab7f0b8893091cccd4d04a3b4210f5a1f6db55 100644
--- a/LayoutTests/http/tests/serviceworker/update.html
+++ b/LayoutTests/http/tests/serviceworker/update.html
@@ -24,8 +24,7 @@ promise_test(function(t) {
                         'active should exist in the initial state');
 
           // A new worker should be found.
-          registration.update();
-          return wait_for_update(t, registration);

[zino, 2015/08/03 13:43:56]

Looks good to me but would you please clarify one more thing?
Can we remove the |wait_for_update()| function?
If so, please remove it.
Otherwise, l-g-t-m.

[jungkees, 2015/08/05 02:09:31]

wait_for_update() helper function is more generally used for a control to wait
until the ServiceWorkerRegistration object is dispatched "updatefound" event,
which actually means almost the samething as registration.update() promise would
imply. But a thing is that "updatefound" is dispatched for the other *update*
scenarios including .register() and UA-initiated updates. So, retaining the
helper function would be useful for those contexts I guess. E.g.
registration-service-worker-attributes.html now uses the function to catch the
"updatefound" event for .register() steps. Thanks for review.

+          return registration.update();
         })
       .then(function() {
           assert_equals(registration.installing.scriptURL, expected_url,