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

chrome/browser/resources/md_bookmarks/item.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-item',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
@@ skipping 22 matching lines @@
 
   observers: [
     'updateFavicon_(item_.url)',
   ],
 
   listeners: {
     'click': 'onClick_',
     'dblclick': 'onDblClick_',
   },
 
+  /** @override */
   attached: function() {
     this.watch('item_', function(store) {
       return store.nodes[this.itemId];
     }.bind(this));
     this.watch('isSelectedItem_', function(store) {
       return !!store.selection.items[this.itemId];
     }.bind(this));
 
     this.updateFromStore();
   },
 
+  /** @return {BookmarksItemElement} */
+  getDropTarget: function() {
+    return this;
+  },
+
   /**
    * @param {Event} e
    * @private
    */
   onMenuButtonOpenClick_: function(e) {
     e.stopPropagation();
     this.fire('open-item-menu', {
       target: e.target,
       item: this.item_,
     });
@@ skipping 33 matching lines @@
   },
 
   /**
    * @param {string} url
    * @private
    */
   updateFavicon_: function(url) {
     this.$.icon.style.backgroundImage = cr.icon.getFavicon(url);
   },
 });