[BackgroundSync] Only allow SyncManager.register to occur from main frame

content/test/data/background_sync/register_sync.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.
+
+'use strict';
+
+function receiveMessage() {
+  return new Promise(function(resolve) {
+    navigator.serviceWorker.addEventListener('message', function(message) {
+      resolve(message.data);
+    });
+  });
+}
+
+navigator.serviceWorker.register('register_sync_sw.js')
+  .then(function() {
+    return receiveMessage();
+  })
+  .then(function(message) {
+    parent.postMessage(message, '*');
+  });