ManagedBookmarkTree = function() {
  // Maps a string id to its Node. Used to lookup an entry by ID.
  this._nodesMap = {};
  this._root = new Node(this._nodesMap, '0', '');
  this._root.appendChild(new Node(this._nodesMap, '1', 'Bookmarks Bar'));
  this._root.appendChild(new Node(this._nodesMap, '2', 'Other Bookmarks'));
}
Tracks all the managed bookmarks, and persists the known state so that
managed bookmarks can be updated after restarts.