Make NotificationEvent constructable.

LayoutTests/http/tests/notifications/resources/serviceworker-notification-event.js

Index: LayoutTests/http/tests/notifications/resources/serviceworker-notification-event.js
diff --git a/LayoutTests/http/tests/notifications/resources/serviceworker-notification-event.js b/LayoutTests/http/tests/notifications/resources/serviceworker-notification-event.js
index 55d8e2226663d49aba54a4192672287ff55dcd61..1788eed555717e267dc5e5e0531b993d2f07b48c 100644
--- a/LayoutTests/http/tests/notifications/resources/serviceworker-notification-event.js
+++ b/LayoutTests/http/tests/notifications/resources/serviceworker-notification-event.js
@@ -4,7 +4,12 @@ importScripts('/resources/testharness-helpers.js');
 test(function() {
     assert_true('NotificationEvent' in self);
 
-}, 'NotificationEvent is exposed.');
+    var event = new NotificationEvent('NotificationEvent');
+    assert_equals(event.type, 'NotificationEvent');
+    assert_own_property(event, 'notification');
+    assert_inherits(event, 'waitUntil');
+
+}, 'NotificationEvent is exposed, and has the expected interface.');
 
 test(function() {
     assert_own_property(self, 'onnotificationclick', 'The notificationclick event exists.');