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

chrome/browser/resources/md_bookmarks/reducers.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.
 
 /**
  * @fileoverview Module of functions which produce a new page state in response
  * to an action. Reducers (in the same sense as Array.prototype.reduce) must be
  * pure functions: they must not modify existing state objects, or make any API
  * calls.
  */
@@ skipping 238 matching lines @@
             SelectedFolderState.isAncestorOf(
                 nodes, action.id, selectedFolder)) {
           return action.id;
         }
         return selectedFolder;
       case 'finish-search':
         return null;
       case 'clear-search':
         // TODO(tsergeant): Return to the folder that was selected before the
         // search.
-        return nodes['0'].children[0];
+        return nodes[bookmarks.util.ROOT_NODE_ID].children[0];
       default:
         return selectedFolder;
     }
   };
 
   var ClosedFolderState = {};
 
   /**
    * @param {ClosedFolderState} closedFolders
    * @param {string|undefined} id
@@ skipping 73 matching lines @@
 
   return {
     reduceAction: reduceAction,
     ClosedFolderState: ClosedFolderState,
     NodeState: NodeState,
     SearchState: SearchState,
     SelectedFolderState: SelectedFolderState,
     SelectionState: SelectionState,
   };
 });