Added missing assets for selected buttons in Files.app's photo editor.

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;
 }
@@ skipping 648 matching lines @@
     url('../images/gallery/icon_rotate_left_selected.png') 1x,
     url('../images/gallery/2x/icon_rotate_left_selected.png') 2x);
 }
 
 .gallery > .toolbar button.undo {
   background-image: -webkit-image-set(
     url('../images/gallery/icon_undo.png') 1x,
     url('../images/gallery/2x/icon_undo.png') 2x);
 }
 
+.gallery > .toolbar button.undo:active,
+.gallery > .toolbar button.undo[pressed] {
+  background-image: -webkit-image-set(
+    url('../images/gallery/icon_undo_selected.png') 1x,
+    url('../images/gallery/2x/icon_undo_selected.png') 2x);
+}
+
 .gallery > .toolbar button.redo {
   background-image: -webkit-image-set(
     url('../images/gallery/icon_redo.png') 1x,
     url('../images/gallery/2x/icon_redo.png') 2x);
   position: absolute;  /* Exclude from center-packing*/
 }
 
+.gallery > .toolbar button.redo:active,
+.gallery > .toolbar button.redo[pressed] {
+  background-image: -webkit-image-set(
+    url('../images/gallery/icon_redo_selected.png') 1x,
+    url('../images/gallery/2x/icon_redo_selected.png') 2x);
+}
+
 .gallery > .toolbar button[disabled] {
   opacity: 0.5;
   pointer-events: none;
 }
 
 .gallery > .toolbar button[hidden] {
   display: none;
 }
 
 .gallery[mode='slide'] > .toolbar > button.mode {
@@ skipping 596 matching lines @@
     url('../images/gallery/slideshow-pause.png') 1x,
     url('../images/gallery/2x/slideshow-pause.png') 2x);
 }
 
 .slideshow-toolbar > .slideshow-end {
   background-image: -webkit-image-set(
     url('../images/gallery/slideshow-end.png') 1x,
     url('../images/gallery/2x/slideshow-end.png') 2x);
   margin-left: -2px;
 }