Fixed the border of the bottom panel of the File Manager.

chrome/browser/resources/file_manager/css/file_manager.css

 /*
  * Copyright (c) 2011 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.
  */
 
 /* Outer frame of the dialog. */
 body {
    -webkit-box-flex: 1;
    -webkit-box-orient: vertical;
@@ skipping 254 matching lines @@
 /* The cr.ui.Table representing the detailed file list. */
 .detail-table {
   position: absolute;
   top: 0;
   left: 0;
   border: 0;
 }
 
 /* Bottom pane describing current selection. */
 .preview-panel {
-  -webkit-border-start: 1px #aaa solid;
   -webkit-box-orient: horizontal;
   display: -webkit-box;
   height: 61px;
   background-color: #F0F0F0;
   padding: 0px 14px 0px 14px;
 
   opacity: 1;
   -webkit-transform: translate(0, 0);
 }
 
@@ skipping 312 matching lines @@
 /* Overlay pane covering the entire file manager window (e.g. image editor)*/
 .overlay-pane {
   position: absolute;
   top: 0;
   left: 0;
   width: 100%;
   height: 100%;
   border: none;
   z-index: 100;
 }