Fix file manager labels and aria role.

ui/webui/resources/js/cr/ui/tree.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 cr.define('cr.ui', function() {
   // require cr.ui.define
   // require cr.ui.limitInputWidth
 
   /**
    * The number of pixels to indent per level.
@@ skipping 39 matching lines @@
      */
     decorate: function() {
       // Make list focusable
       if (!this.hasAttribute('tabindex'))
         this.tabIndex = 0;
 
       this.addEventListener('click', this.handleClick);
       this.addEventListener('mousedown', this.handleMouseDown);
       this.addEventListener('dblclick', this.handleDblClick);
       this.addEventListener('keydown', this.handleKeyDown);
+
+      this.setAttribute('role', 'group');
     },
 
     /**
      * Returns the tree item that are children of this tree.
      */
     get items() {
       return this.children;
     },
 
     /**
@@ skipping 618 matching lines @@
     }
     return item;
   }
 
   // Export
   return {
     Tree: Tree,
     TreeItem: TreeItem
   };
 });