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

content/test/data/background_sync/register_sync_sw.js

Index: content/test/data/background_sync/register_sync_sw.js
diff --git a/content/test/data/background_sync/register_sync_sw.js b/content/test/data/background_sync/register_sync_sw.js
new file mode 100644
index 0000000000000000000000000000000000000000..da7ff4cfc3ea89a961c9338ee0b5ab3d61e447c5
--- /dev/null
+++ b/content/test/data/background_sync/register_sync_sw.js
@@ -0,0 +1,22 @@
+// 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');
+    });
+});