[MD Bookmarks] Implement adding folders and bookmarks from toolbar menu.

chrome/browser/resources/md_bookmarks/folder_node.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/color.html">
 <link rel="import" href="chrome://bookmarks/icons.html">
 <link rel="import" href="chrome://bookmarks/shared_style.html">
 
 <dom-module id="bookmarks-folder-node">
   <template>
@@ skipping 50 matching lines @@
         padding: 8px;
         width: 36px;
       }
     </style>
 
     <div id="container" class="v-centered">
       <div id="folder-label" class="v-centered" on-tap="selectFolder_">
         <iron-icon icon="[[getFolderIcon_(isSelectedFolder)]]"></iron-icon>
         <div class="menu-label">[[item.title]]</div>
       </div>
-      <template is="dom-if" if="[[hasChildFolder_(item.children)]]">
+      <template is="dom-if" if="[[hasChildFolder_(item.children.length)]]">
         <paper-icon-button icon="[[getArrowIcon_(item.isOpen)]]"
             on-tap="toggleFolder_"></paper-icon-button>
       </template>
     </div>
     <div id="descendants" hidden$="[[!item.isOpen]]">
       <template is="dom-repeat" items="[[item.children]]"
           filter="isFolder_" observe="url" as="child">
         <bookmarks-folder-node item="[[child]]"
             is-selected-folder="[[child.isSelectedFolder]]">
         </bookmarks-folder-node>
       </template>
     </div>
   </template>
   <script src="chrome://bookmarks/folder_node.js"></script>
 </dom-module>