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

chrome/test/data/webui/md_bookmarks/app_test.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.
 
 suite('<bookmarks-app>', function() {
   var app;
   var store;
 
   function resetStore() {
     store = new bookmarks.TestStore({});
@@ skipping 20 matching lines @@
 
     app = document.createElement('bookmarks-app');
     replaceBody(app);
   });
 
   teardown(function() {
     window.localStorage.clear();
   });
 
   test('write and load closed folder state', function() {
-    var closedFolders = {'1': true};
+    var closedFoldersList = ['1'];
+    var closedFolders = new Set(closedFoldersList);
     store.data.closedFolders = closedFolders;
     store.notifyObservers();
 
     // Ensure closed folders are written to local storage.
     assertDeepEquals(
-        JSON.stringify(closedFolders),
+        JSON.stringify(Array.from(closedFolders)),
         window.localStorage[LOCAL_STORAGE_CLOSED_FOLDERS_KEY]);
 
     resetStore();
     app = document.createElement('bookmarks-app');
     replaceBody(app);
 
     // Ensure closed folders are read from local storage.
-    assertDeepEquals(closedFolders, store.data.closedFolders);
+    assertDeepEquals(closedFoldersList, normalizeSet(store.data.closedFolders));
   });
 });