Uses the activedescendant_changed event received from Blink to fire the right focus event

content/test/data/accessibility/event/menulist-next.html

Index: content/test/data/accessibility/event/menulist-next.html
diff --git a/content/test/data/accessibility/event/menulist-next.html b/content/test/data/accessibility/event/menulist-next.html
index 98d7f3329ab7b025287718150dd3a9841955b56e..61ea0676f17269cace69cc82431e1c2cd7c402c7 100644
--- a/content/test/data/accessibility/event/menulist-next.html
+++ b/content/test/data/accessibility/event/menulist-next.html
@@ -10,7 +10,12 @@
   <option>Banana</option>
 </select>
 <script>
-  document.querySelector('select').focus();
+  var comboBox = document.querySelector('select');
+  comboBox.focus();
+  // Expand the combo box.
+  var clickEvent = document.createEvent('MouseEvents');
+  clickEvent.initMouseEvent('mousedown', true, true, window);
+  comboBox.dispatchEvent(clickEvent);
   function go() {
     document.querySelector('select').selectedIndex = 1;
   }