Make Menu.findMenuItem_ return the first MenuItem.

This fixes the regression in http://crbug.com/480381 where 'activate'
event is not fired for menu items under a grouping div rather than
being direct children of a Menu. Also fixes js error when hovering
such menu items.

BUG=480381

Review URL: https://codereview.chromium.org/1095303004

Cr-Commit-Position: refs/heads/master@{#327080}

ui/webui/resources/js/cr/ui/menu.js

@@ -88,7 +88,7 @@ cr.define('cr.ui', function() {
      * @private
      */
     findMenuItem_: function(node) {
-      while (node && node.parentNode != this) {
+      while (node && node.parentNode != this && !(node instanceof MenuItem)) {
         node = node.parentNode;
       }
       return node ? assertInstanceof(node, MenuItem) : null;