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 3557fd03845df0763a44ed268928e54671403103..35beaec93d8228861edf12291719d7b559e066c3 100644
--- a/chrome/browser/resources/engagement/site_engagement.js
+++ b/chrome/browser/resources/engagement/site_engagement.js
@@ -4,6 +4,27 @@
 
 '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 pagePopulatedCallback = null;
+var isPagePopulated = false;
+
+function setPagePopulatedCallbackForTest(callback) {
+  if (isPagePopulated) {
+    callback();
+  } else {
+    pagePopulatedCallback = callback;
+  }
+}
+
+function notifyPagePopulated() {
+  isPagePopulated = true;
+  if (pagePopulatedCallback) {
+    pagePopulatedCallback();
+    pagePopulatedCallback = null;
+  }
+}
+
 define('main', [
     'chrome/browser/ui/webui/engagement/site_engagement.mojom',
     'content/public/renderer/frame_interfaces',
@@ -155,6 +176,8 @@ define('main', [
         info.score = Number(Math.round(info.score * 100) / 100);
         engagementTableBody.appendChild(createRow(info));
       });
+
+      notifyPagePopulated();
     }
 
     /**