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

chrome/browser/resources/md_bookmarks/util.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.
 
 /**
  * @fileoverview Utility functions for the Bookmarks page.
  */
 
 cr.define('bookmarks.util', function() {
   /**
@@ skipping 36 matching lines @@
     }
 
     return nodeList;
   }
 
   /** @return {!BookmarksPageState} */
   function createEmptyState() {
     return {
       nodes: {},
       selectedFolder: '0',
-      closedFolders: {},
+      closedFolders: new Set(),
       search: {
         term: '',
         inProgress: false,
         results: [],
       },
       selection: {
-        items: {},
+        items: new Set(),
         anchor: null,
       },
     };
   }
 
   /**
    * @param {BookmarksPageState} state
    * @return boolean
    */
   function isShowingSearch(state) {
@@ skipping 15 matching lines @@
   }
 
   return {
     createEmptyState: createEmptyState,
     getDisplayedList: getDisplayedList,
     hasChildFolders: hasChildFolders,
     isShowingSearch: isShowingSearch,
     normalizeNodes: normalizeNodes,
   };
 });