Wait for directory tree children before clicking the expand icon

Similar to the issue 891197 fix, wait for the directory tree row
attribute [has-children=true] before clicking the expand icon.

Bug: 899664
Change-Id: I26d0f96feba85b6bd0cd9d03b76b438f1b1b7f0b
Reviewed-on: https://chromium-review.googlesource.com/c/1321649Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605903}

ui/file_manager/integration_tests/file_manager/create_new_folder.js

@@ -142,7 +142,8 @@ function createNewFolder(appId, initialEntrySet, selector) {
  * @return {Promise} Promise fulfilled on success.
  */
 function expandRoot(appId, selector) {
-  const expandIcon = selector + ' > .tree-row > .expand-icon';
+  const expandIcon =
+      selector + ' > .tree-row[has-children=true] > .expand-icon';
 
   return new Promise(function(resolve) {
     // Wait for the subtree expand icon to appear.