[MD Bookmarks] Add a drag and drop indicator to bookmarks.

chrome/browser/resources/md_bookmarks/folder_node.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-folder-node',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
@@ skipping 20 matching lines @@
 
     /** @private */
     isSelectedFolder_: {
       type: Boolean,
       value: false,
       reflectToAttribute: true,
       computed: 'computeIsSelected_(itemId, selectedFolder_)'
     },
   },
 
+  /** @override */
   attached: function() {
     this.watch('item_', function(state) {
       return state.nodes[this.itemId];
     }.bind(this));
     this.watch('isClosed_', function(state) {
       return !!state.closedFolders[this.itemId];
     }.bind(this));
     this.watch('selectedFolder_', function(state) {
       return state.selectedFolder;
     });
 
     this.updateFromStore();
   },
 
+  /** @return {HTMLElement} */
+  getDropTarget: function() {
+    return this.$.container;
+  },
+
   /**
    * @private
    * @return {string}
    */
   getFolderIcon_: function() {
     return this.isSelectedFolder_ ? 'bookmarks:folder-open' : 'cr:folder';
   },
 
   /** @private */
   selectFolder_: function() {
@@ skipping 19 matching lines @@
    */
   computeIsSelected_: function(itemId, selectedFolder) {
     return itemId == selectedFolder;
   },
 
   /**
    * @private
    * @return {boolean}
    */
   hasChildFolder_: function() {
-    for (var i = 0; i < this.item_.children.length; i++) {
-      if (this.isFolder_(this.item_.children[i]))
-        return true;
-    }
-    return false;
+    return bookmarks.util.hasChildFolders(this.itemId, this.getState().nodes);
   },
 
   /** @private */
   depthChanged_: function() {
     this.style.setProperty('--node-depth', String(this.depth));
   },
 
   /**
    * @private
    * @return {number}
    */
   getChildDepth_: function() {
     return this.depth + 1;
   },
 
   /**
    * @param {string} itemId
    * @private
    * @return {boolean}
    */
   isFolder_: function(itemId) {
     return !this.getState().nodes[itemId].url;
-  }
+  },
+
+  /**
+   * @private
+   * @return {boolean}
+   */
+  isRootFolder_: function() {
+    return this.depth == 0;
+  },
 });