Make body background black to mitigate flash on hitting full screen for gallery.

ui/file_manager/gallery/css/gallery.css

 /* Copyright (c) 2014 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. */
 
 body {
   -webkit-user-select: none;
+  background-color: black;
   font-size: 84%;
   margin: 0;
 }
 
 button {
   cursor: pointer;
 }
 
 button:focus {
   outline: 1px solid rgb(77, 144, 254);
@@ skipping 1345 matching lines @@
 .thumbnail-view .animation-thumbnail {
   background-repeat: no-repeat;
   background-size: cover;
   display: none;
   position: absolute;
 }
 
 .thumbnail-view .animation-thumbnail.animating {
   display: block;
 }