Bug 1498713 - Use .children for context menu test helper instead of .childNodes to fix tests in browser.xhtml;r=bdahl

Differential Revision: https://phabricator.services.mozilla.com/D8604

--HG--
extra : moz-landing-system : lando

browser/base/content/test/general/contextmenu_common.js

@@ -36,8 +36,8 @@ function closeContextMenu() {
 function getVisibleMenuItems(aMenu, aData) {
     var items = [];
     var accessKeys = {};
-    for (var i = 0; i < aMenu.childNodes.length; i++) {
+    for (var i = 0; i < aMenu.children.length; i++) {
-        var item = aMenu.childNodes[i];
+        var item = aMenu.children[i];
         if (item.hidden)
             continue;