HiDPI assets for Chrome OS Files app

chrome/browser/resources/file_manager/css/gallery.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. */
 
 body {
   -webkit-user-select: none;
   font-family: Open Sans, Droid Sans Fallback, sans-serif;
   font-size: 84%;
   margin: 0;
 }
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 }
 
 .gallery .toolbar .ribbon.fade-left.fade-right {
   -webkit-mask-image: -webkit-linear-gradient(left, rgba(0, 0, 0, 0) 0,
                                                     rgba(0, 0, 0, 1) 40px,
                                                     rgba(0, 0, 0, 1) 230px,
                                                     rgba(0, 0, 0, 0) 100%);
 }
 
 .gallery .image-wrapper {
+  background-size: 45px 45px;
   border: 1px solid rgba(0, 0, 0, 0);  /* transparent black */
   height: 45px;
   overflow: hidden;
   position: relative;
   width: 45px;
 }
 
 .gallery .image-wrapper > img {
   position: absolute;
 }
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 .gallery[video] .video-controls {
   -webkit-box-flex: 1;
   display: -webkit-box;
 }
 
 .gallery[video] > .toolbar .button.edit,
 .gallery[video] > .toolbar .edit-bar {
   display: none;
 }