[MD Bookmarks] Allow dragging onto empty bookmark lists.

chrome/browser/resources/md_bookmarks/list.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-list',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
@@ skipping 24 matching lines @@
   attached: function() {
     this.watch('displayedList_', function(state) {
       return bookmarks.util.getDisplayedList(state);
     });
     this.watch('searchTerm_', function(state) {
       return state.search.term;
     });
     this.updateFromStore();
   },
 
+  getDropTarget: function() {
+    return this.$.message;
+  },
+
   /**
    * @param {Event} e
    * @private
    */
   onOpenItemMenu_: function(e) {
     this.menuItem_ = e.detail.item;
     var menu = /** @type {!CrActionMenuElement} */ (
         this.$.dropdown);
     menu.showAt(/** @type {!Element} */ (e.detail.target));
   },
@@ skipping 50 matching lines @@
   /** @private */
   isEmptyList_: function() {
     return this.displayedList_.length == 0;
   },
 
   /** @private */
   deselectItems_: function() {
     this.dispatch(bookmarks.actions.deselectItems());
   },
 });