BackgroundSync sync events need to be ExtendableEvents, content browser tests

content/test/data/background_sync/sync_event_fulfilled.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) {
+    if (!('waitUntil' in event)) {
+      console.log("waitUntil missing on event");
+      return;
+    }
+    event.waitUntil(new Promise(function(resolve, reject) {
+        setTimeout(function() {
+          console.log('Fulfilling onsync event');
+          resolve("Fulfilled");

[michaeln, 2015/06/15 20:34:32]

I don't think the "Fulfilled" value being passed to resolve is used, if not, i'd
suggest removing it to avoid confusion.

[chasej, 2015/06/17 16:18:07]

Done.

+        }, 5);
+    }));
+};