ServiceWorker: Introduce ServiceWorkerMessageEvent to replace MessageEvent (3/3).

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 a09bb986822087ad56702433a35367b5b86b5115..499bedd0c9283ff7b9a37a377b26ca96bb2fa612 100644
--- a/LayoutTests/http/tests/serviceworker/postmessage-to-client.html
+++ b/LayoutTests/http/tests/serviceworker/postmessage-to-client.html
@@ -15,7 +15,7 @@ t.step(function() {
       .then(function() { return with_iframe(scope); })
       .then(function(frame) {
           var w = frame.contentWindow;
-          w.onmessage = t.step_func(onMessage);
+          w.navigator.serviceWorker.onmessage = t.step_func(onMessage);
           w.navigator.serviceWorker.controller.postMessage('ping');
         })
       .catch(unreached_rejection(t));