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

components/sync/driver/resources/user_events.js

+// Copyright 2017 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.
+
+cr.define('chrome.sync.user_events', function() {
+  function write() {
+    chrome.sync.writeUserEvent(
+      $('event-time-usec-input').value,
+      $('navigation-id-input').value);
+  }
+
+  function onLoad() {
+    $('create-event-button').addEventListener('click', write);
+  }
+
+  return {
+    onLoad: onLoad
+  };
+});
+
+document.addEventListener(
+    'DOMContentLoaded',
+    chrome.sync.user_events.onLoad,
+    false);