Make drag image containr invisible.

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

 /* Copyright (c) 2012 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;
    -webkit-transition: opacity 70ms linear;
    -webkit-user-select: none;
@@ skipping 152 matching lines @@
   height: 32px;
   padding-bottom: 8px;
   padding-top: 8px;
 }
 
 /* Main part of the dialog between header and footer. */
 .dialog-container {
   -webkit-box-align: stretch;
   -webkit-box-flex: 1;
   -webkit-box-orient: horizontal;
+  background-color: white;  /* Makes #drag-image-container invisible. */
   display: -webkit-box;
   overflow: hidden;
   position: relative;
 }
 
 /* List/grid and preview are inside this container. */
 .dialog-main {
   -webkit-box-align: stretch;
   -webkit-box-flex: 1;
   -webkit-box-orient: vertical;
@@ skipping 1057 matching lines @@
   top: 0;
   width: 40px;
 }
 
 .unmounted-panel:not([loading]) > .gdata.loading,
 .unmounted-panel:not([error]) > .gdata.error,
 .unmounted-panel:not([retry]) > .gdata.retry,
 .unmounted-panel:not([retry]) > .gdata.learn-more {
   display: none;
 }