[Sync] Add a simple UI to sync-internals to create UserEvents.

components/sync/driver/resources/sync_index.js

 // Copyright (c) 2011 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.
 
 // Allow platform specific CSS rules.
 //
 // TODO(akalin): BMM and options page does something similar, too.
 // Move this to util.js.
 if (cr.isWindows)
   document.documentElement.setAttribute('os', 'win');
 
 cr.ui.decorate('tabbox', cr.ui.TabBox);
+
+cr.define('chrome.sync.index', function() {
+  function setUserEventsTabVisibility(visible) {
+    $('sync-user-events-tab').style.display = visible ? 'block' : 'none';

[Dan Beam, 2017/05/16 22:40:54]

$('sync-user-events-tab').hidden = !visible;

[skym, 2017/05/22 21:55:48]

This doesn't work, see
https://codereview.chromium.org/2872023002/diff/40001/components/sync/driver/...
for my explanation of why not.

+  }
+
+  function onLoad() {
+    chrome.sync.userEventsVisibilityCallback = setUserEventsTabVisibility;
+    chrome.sync.requestUserEventsVisibility();
+  }
+
+  return {
+    onLoad: onLoad
+  };
+});
+
+document.addEventListener('DOMContentLoaded', chrome.sync.index.onLoad, false);