Add a browser test for the chrome://site-engagement WebUI.

chrome/browser/resources/engagement/site_engagement.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';
 
+// Allow a function to be provided by tests, which will be called when
+// the page has been populated with site engagement details.
+var resolvePageIsPopulated = null;
+var pageIsPopulated = new Promise(function(resolve, reject) {
+  resolvePageIsPopulated = resolve;
+});
+
+function pageIsPopulatedPromiseForTest() {
+  return pageIsPopulated;
+}
+
 define('main', [
     'chrome/browser/engagement/site_engagement.mojom',
     'content/public/renderer/frame_interfaces',
 ], function(siteEngagementMojom, frameInterfaces) {
   return function() {
     var uiHandler = new siteEngagementMojom.SiteEngagementUIHandlerPtr(
         frameInterfaces.getInterface(
             siteEngagementMojom.SiteEngagementUIHandler.name));
 
     var engagementTableBody = $('engagement-table-body');
@@ skipping 131 matching lines @@
      * Regenerates the engagement table from |info|.
      */
     function renderTable() {
       clearTable();
       sortInfo();
       // Round each score to 2 decimal places.
       info.forEach(function(info) {
         info.score = Number(Math.round(info.score * 100) / 100);
         engagementTableBody.appendChild(createRow(info));
       });
+
+      resolvePageIsPopulated();
     }
 
     /**
      * Retrieve site engagement info and render the engagement table.
      */
     function updateEngagementTable() {
       // Populate engagement table.
       uiHandler.getSiteEngagementInfo().then(function(response) {
         info = response.info;
         renderTable(info);
       });
     };
 
     updateEngagementTable();
     enableAutoupdate();
   };
 });