[MD Bookmarks] Clear drop indicator when dragging over invalid target.

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 459 matching lines @@
       var overElement = getBookmarkElement(e.path);
       this.autoExpander_.update(e, overElement);
       if (!overElement)
         return;
 
 
       // Now we know that we can drop. Determine if we will drop above, on or
       // below based on mouse position etc.
       this.dropDestination_ =
           this.calculateDropDestination_(e.clientY, overElement);
-      if (!this.dropDestination_)
+      if (!this.dropDestination_) {
+        this.dropIndicator_.finish();
         return;
+      }
 
       if (e.dataTransfer) {
         e.dataTransfer.dropEffect =
             this.dragInfo_.isSameProfile() ? 'move' : 'copy';
       }
 
       this.dropIndicator_.update(this.dropDestination_);
     },
 
     /**
@@ skipping 143 matching lines @@
       this.dropIndicator_.setTimeout = this.setTimeout_;
     }
   };
 
   return {
     DNDManager: DNDManager,
     DragInfo: DragInfo,
     DropIndicator: DropIndicator,
   };
 });