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

chrome/browser/resources/md_bookmarks/app.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-app',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
 
   properties: {
     /** @private */
     searchTerm_: {
       type: String,
       observer: 'searchTermChanged_',
     },
 
+    /** @type {ClosedFolderState} */
     closedFoldersState_: {
       type: Object,
       observer: 'closedFoldersStateChanged_',
     },
 
     /** @private */
     sidebarWidth_: String,
   },
 
   /** @private{?function(!Event)} */
@@ skipping 13 matching lines @@
     });
 
     chrome.bookmarks.getTree(function(results) {
       var nodeList = bookmarks.util.normalizeNodes(results[0]);
       var initialState = bookmarks.util.createEmptyState();
       initialState.nodes = nodeList;
       initialState.selectedFolder = nodeList[ROOT_NODE_ID].children[0];
       var closedFoldersString =
           window.localStorage[LOCAL_STORAGE_CLOSED_FOLDERS_KEY];
       initialState.closedFolders = closedFoldersString ?
-          /** @type {!Object<string,boolean>} */ (
-              JSON.parse(closedFoldersString)) :
-          {};
+          new Set(
+              /** @type Array<string> */ (JSON.parse(closedFoldersString))) :
+          new Set();
 
       bookmarks.Store.getInstance().init(initialState);
       bookmarks.ApiListener.init();
 
     }.bind(this));
 
     this.boundUpdateSidebarWidth_ = this.updateSidebarWidth_.bind(this);
 
     this.initializeSplitter_();
 
@@ skipping 45 matching lines @@
       var ids = results.map(function(node) {
         return node.id;
       });
       this.dispatch(bookmarks.actions.setSearchResults(ids));
     }.bind(this));
   },
 
   /** @private */
   closedFoldersStateChanged_: function() {
     window.localStorage[LOCAL_STORAGE_CLOSED_FOLDERS_KEY] =
-        JSON.stringify(this.closedFoldersState_);
+        JSON.stringify(Array.from(this.closedFoldersState_));
   },
 });