[MD Bookmarks] Change selection items from Map to Set.

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.
 
 cr.define('bookmarks', function() {
   /**
    * @param {BookmarkElement} element
    * @return {boolean}
    */
   function isBookmarkItem(element) {
@@ skipping 310 matching lines @@
      * @private
      * @param {Event} e
      */
     onDragStart_: function(e) {
       var dragElement = getBookmarkElement(e.path);
       if (!dragElement)
         return;
 
       // Determine the selected bookmarks.
       var state = bookmarks.Store.getInstance().data;
-      var draggedNodes = Object.keys(state.selection.items);
+      var draggedNodes = Array.from(state.selection.items);
 
       if (isBookmarkFolderNode(dragElement) ||
           draggedNodes.indexOf(dragElement.itemId) == -1) {
         // TODO(calamity): clear current selection.
         draggedNodes = [dragElement.itemId];
       }
 
       e.preventDefault();
 
       // If we are dragging a single link, we can do the *Link* effect.
@@ skipping 147 matching lines @@
 
       var validDropPositions = DropPosition.NONE;
 
       // Cannot drop above if the item above is already in the drag source.
       var previousElem = overElement.previousElementSibling;
       if (!previousElem || !dragInfo.isDraggingBookmark(previousElem.itemId))
         validDropPositions |= DropPosition.ABOVE;
 
       // Don't allow dropping below an expanded sidebar folder item since it is
       // confusing to the user anyway.
-      if (isOverFolderNode && !state.closedFolders[overElement.itemId] &&
+      if (isOverFolderNode && !state.closedFolders.has(overElement.itemId) &&
           bookmarks.util.hasChildFolders(overElement.itemId, state.nodes)) {
         return validDropPositions;
       }
 
       var nextElement = overElement.nextElementSibling;
 
       // The template element sits past the end of the last bookmark element.
       if (!isBookmarkItem(nextElement) && !isBookmarkFolderNode(nextElement))
         nextElement = null;
 
@@ skipping 20 matching lines @@
       return !this.dragInfo_.isDraggingChildBookmark(overElement.itemId)
     },
   };
 
   return {
     DNDManager: DNDManager,
     DragInfo: DragInfo,
     DropIndicator: DropIndicator,
   };
 });