Service worker: Update usecounter.html test.

This test is flaky with the ServiceWorker servicification
More details in https://crbug.com/808964.

The previous test was checking the UseCounter immediatly after a new
window loads. It is racy because the UseCounter state is sent in
ServiceWorkerProviderHost::SendSetControllerServiceWorker(). In the case
of a very slow browser process, it can happens after the new window
loads.

Instead of checking the UseCounter state after a window loads, it uses
an observer to wait until its state is updated.

Bug: 808964
Change-Id: I53c2bf2775a837a3a4b0c6cf9b196f26fe7b7f5e
Reviewed-on: https://chromium-review.googlesource.com/901610Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534667}

third_party/WebKit/LayoutTests/http/tests/serviceworker/chromium/usecounter.html

@@ -87,9 +87,14 @@ promise_test(t => {
           return openWindow(kScope);
         })
       .then(win => {
-          assert_true(isUseCounted(win, kFeature));
-          assert_true(isUseCounted(win, kDeprecatedFeature));
-        });
+        // Check UseCounters have been sent to the new window. Since this is can
+        // happen after the new window's document load, an observer is used to
+        // wait until it happens.
+        return Promise.all([
+            observeUseCounter(win, kFeature),
+            observeUseCounter(win, kDeprecatedFeature)
+        ]);
+      })
   }, 'UseCounter on ServiceWorkerGlobalScope');
 
 promise_test(t => {