[MD Bookmarks] Flatten sidebar.

chrome/browser/resources/md_bookmarks/sidebar.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-sidebar',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
 
   properties: {
-    rootFolders: Array,
+    folders: Array,
   },
 
   attached: function() {
-    this.observe('rootFolders', function(store) {
-      return store.nodes['0'].children;
+    this.observe('folders', function(store) {
+      return store.sidebar.folders;
     });
     this.updateFromStore();
   },
+
+  isVisible_: function(folder) {
+    return folder.visible;
+  },
 });