Don't rely on untrusted events in site isolation browser test.

content/test/data/site_isolation/page-with-select.html

Index: content/test/data/site_isolation/page-with-select.html
diff --git a/content/test/data/site_isolation/page-with-select.html b/content/test/data/site_isolation/page-with-select.html
index c8282d4fd386e3178e10f0a449f49054d4517f8a..465910570c7d4721569bf840564ae8e23999e1e9 100644
--- a/content/test/data/site_isolation/page-with-select.html
+++ b/content/test/data/site_isolation/page-with-select.html
@@ -10,10 +10,8 @@ select {
 </style>
 <script>
 
-function openSelectMenu(target) {
-  var evt = document.createEvent("MouseEvents");
-  evt.initMouseEvent("mousedown", true, true, window);
-  document.querySelector('select').dispatchEvent(evt);
+function focusSelectMenu() {
+  document.querySelector('select').focus();
 }
 
 </script>
@@ -25,4 +23,4 @@ function openSelectMenu(target) {
   <option>Banana</option>
 </select>
 </body>
-</html>
+</html>