[MD Bookmarks] Change selection items from Map to Set.

chrome/browser/resources/md_bookmarks/folder_node.js

 // Copyright 2016 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.
 
 Polymer({
   is: 'bookmarks-folder-node',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
@@ skipping 26 matching lines @@
       computed: 'computeIsSelected_(itemId, selectedFolder_)'
     },
   },
 
   /** @override */
   attached: function() {
     this.watch('item_', function(state) {
       return state.nodes[this.itemId];
     }.bind(this));
     this.watch('isClosed_', function(state) {
-      return !!state.closedFolders[this.itemId];
+      return state.closedFolders.has(this.itemId);
     }.bind(this));
     this.watch('selectedFolder_', function(state) {
       return state.selectedFolder;
     });
 
     this.updateFromStore();
   },
 
   /** @return {HTMLElement} */
   getDropTarget: function() {
@@ skipping 65 matching lines @@
   },
 
   /**
    * @private
    * @return {boolean}
    */
   isRootFolder_: function() {
     return this.depth == 0;
   },
 });