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

chrome/browser/resources/engagement/site_engagement.js

Index: chrome/browser/resources/engagement/site_engagement.js
diff --git a/chrome/browser/resources/engagement/site_engagement.js b/chrome/browser/resources/engagement/site_engagement.js
index f8c7ccccd9aa492e39e7f14c040a9ce41733ed15..2e66df9ea817a05835feb538007fb743aa83d45c 100644
--- a/chrome/browser/resources/engagement/site_engagement.js
+++ b/chrome/browser/resources/engagement/site_engagement.js
@@ -4,6 +4,17 @@
 
 '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',
@@ -155,6 +166,8 @@ define('main', [
         info.score = Number(Math.round(info.score * 100) / 100);
         engagementTableBody.appendChild(createRow(info));
       });
+
+      resolvePageIsPopulated();
     }
 
     /**