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

content/test/data/background_sync/register_sync_sw.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 sendMessageToClients(message) {
+  clients.matchAll({ includeUncontrolled: true }).then(function(clients) {
+    clients.forEach(function(client) {
+      client.postMessage(message);
+    });
+  });
+}
+
+self.addEventListener('activate', function(event) {
+  registration.sync.register('foo')
+    .then(function () {
+      sendMessageToClients('registration succeeded');
+    }, function(e) {
+      sendMessageToClients('registration failed');
+    });
+});