BackgroundSync sync events need to be ExtendableEvents, content browser tests

content/test/data/background_sync/sync_event_interface.js

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+this.onsync = function(event) {
+    var eventProperties = [
+      // Extract name from toString result: "[object <Class>]"
+      Object.prototype.toString.call(event).match(/\s([a-zA-Z]+)/)[1],
+      (typeof event.waitUntil)
+    ];
+
+    console.log(eventProperties.join('|'));
+};