Add ServiceWorker InstallPhaseEvent.waitUntil() layout test.

LayoutTests/http/tests/serviceworker/postmessage-to-client.html

Index: LayoutTests/http/tests/serviceworker/postmessage-to-client.html
diff --git a/LayoutTests/http/tests/serviceworker/postmessage-to-client.html b/LayoutTests/http/tests/serviceworker/postmessage-to-client.html
index a1718ad4970f3d3af2588ce647f6b3eedc3f60c8..236905ba76cd5a6a24df73d08c973353838f3670 100644
--- a/LayoutTests/http/tests/serviceworker/postmessage-to-client.html
+++ b/LayoutTests/http/tests/serviceworker/postmessage-to-client.html
@@ -12,7 +12,7 @@ t.step(function() {
 
     function onRegister(worker) {
         worker.addEventListener('statechange', t.step_func(function(event) {
-            if (event.target.state == 'active')
+            if (event.target.state == 'activated')
                 onActive();
         }));
     }