[MD Bookmarks] Persist collapsed folders between page loads.

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,
   ],
@@ skipping 18 matching lines @@
   /** @override */
   attached: function() {
     this.watch('searchTerm_', function(store) {
       return store.search.term;
     });
 
     chrome.bookmarks.getTree(function(results) {
       var nodeList = bookmarks.util.normalizeNodes(results[0]);
       var initialState = bookmarks.util.createEmptyState();
       initialState.nodes = nodeList;
-      initialState.selectedFolder =
-          nodeList[bookmarks.util.ROOT_NODE_ID].children[0];
+      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)) :
+          {};
 
       bookmarks.Store.getInstance().init(initialState);
       bookmarks.ApiListener.init();
 
     }.bind(this));
 
     this.boundUpdateSidebarWidth_ = this.updateSidebarWidth_.bind(this);
 
     this.initializeSplitter_();
 
@@ skipping 41 matching lines @@
       return;
 
     chrome.bookmarks.search(this.searchTerm_, function(results) {
       var ids = results.map(function(node) {
         return node.id;
       });
       this.dispatch(bookmarks.actions.setSearchResults(ids));
     }.bind(this));
   },
 });