BackgroundSync sync events need to be ExtendableEvents, content browser tests

content/test/data/background_sync/sync_event_rejected.js

Index: content/test/data/background_sync/sync_event_rejected.js
diff --git a/content/test/data/background_sync/sync_event_rejected.js b/content/test/data/background_sync/sync_event_rejected.js
new file mode 100644
index 0000000000000000000000000000000000000000..ca697165403279bc42fc75240715ede4fd6972ca
--- /dev/null
+++ b/content/test/data/background_sync/sync_event_rejected.js
@@ -0,0 +1,16 @@
+// 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('Rejecting onsync event');
+          reject();
+        }, 5);
+    }));
+};