[MD Bookmarks] Persist collapsed folders between page loads.

chrome/browser/resources/md_bookmarks/dnd_manager.js

 // Copyright 2017 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.
 
-/**
- * Enumeration of valid drop locations relative to an element. These are
- * bit masks to allow combining multiple locations in a single value.
- * @enum {number}
- * @const
- */
-var DropPosition = {
-  NONE: 0,
-  ABOVE: 1,
-  ON: 2,
-  BELOW: 4,
-};
-
-/** @typedef {{element: BookmarkElement, position: DropPosition}} */
-var DropDestination;
-
 cr.define('bookmarks', function() {
   /**
    * @param {BookmarkElement} element
    * @return {boolean}
    */
   function isBookmarkItem(element) {
     return element.tagName == 'BOOKMARKS-ITEM';
   }
 
   /**
@@ skipping 459 matching lines @@
     /**
      * @private
      * @param {BookmarkElement} overElement
      * @return {DropPosition}
      */
     calculateDropAboveBelow_: function(overElement) {
       var dragInfo = this.dragInfo_;
       var state = bookmarks.Store.getInstance().data;
 
       // We cannot drop between Bookmarks bar and Other bookmarks.
-      if (getBookmarkNode(overElement).parentId == bookmarks.util.ROOT_NODE_ID)
+      if (getBookmarkNode(overElement).parentId == ROOT_NODE_ID)
         return DropPosition.NONE;
 
       var isOverFolderNode = isBookmarkFolderNode(overElement);
 
       // We can only drop between items in the tree if we have any folders.
       if (isOverFolderNode && !dragInfo.isDraggingFolders())
         return DropPosition.NONE;
 
       var validDropPositions = DropPosition.NONE;
 
@@ skipping 38 matching lines @@
       return !this.dragInfo_.isDraggingChildBookmark(overElement.itemId)
     },
   };
 
   return {
     DNDManager: DNDManager,
     DragInfo: DragInfo,
     DropIndicator: DropIndicator,
   };
 });