[MD Bookmarks] Add undo/redo functionality to MD Bookmarks.

chrome/browser/resources/md_bookmarks/constants.js

 // Copyright 2017 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.
 
 /**
  * Enumeration of valid drop locations relative to an element. These are
  * bit masks to allow combining multiple locations in a single value.
  * @enum {number}
  * @const
  */
 var DropPosition = {
   NONE: 0,
   ABOVE: 1,
   ON: 2,
   BELOW: 4,
 };
 
 /**
  * @enum {string}
  * @const
  */
 var Command = {
   EDIT: 'edit',
   COPY: 'copy',
   DELETE: 'delete',
   OPEN_NEW_TAB: 'open-new-tab',
   OPEN_NEW_WINDOW: 'open-new-window',
   OPEN_INCOGNITO: 'open-incognito',
+  UNDO: 'undo',
+  REDO: 'redo',
   // OPEN triggers when you double-click an item.
   OPEN: 'open',
 };
 
 /** @const */
 var LOCAL_STORAGE_CLOSED_FOLDERS_KEY = 'closedState';
 
 /** @const */
 var LOCAL_STORAGE_TREE_WIDTH_KEY = 'treeWidth';
 
 /** @const */
 var ROOT_NODE_ID = '0';