Fix some accessibility issues in image editing.

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;
 }
@@ skipping 803 matching lines @@
 .edit-modal-wrapper .container,
 .edit-modal-wrapper .action-buttons {
   align-items: center;
   display: flex;
   height: 100%;
   width: 100%;
 }
 
 .edit-modal-wrapper .container {
   justify-content: center;
+  left: 0;
+  position: absolute;
+  top: 0;
 }
 
 .edit-modal-wrapper .action-buttons {
   justify-content: flex-end;
-  left: 0;
-  position: absolute;
-  top: 0;
 }
 
 .edit-modal button.edit-toolbar {
   background: transparent;
   border: 0 none;
   border-image: none;
+  border-radius: 2px;
   box-sizing: border-box;
   color: white;
   min-width: 0;
   padding: 0;
   position: relative;
 }
 
+.edit-modal button.edit-toolbar:focus {
+  background-color: rgba(255, 255, 255, 0.12);

[oka, 2016/10/06 09:11:55]

Should we use 
rgba(0, 0, 0, 0.3) for consistency?
The EXIT button in the lower right corner in the edit toolbar is highlighted
with this value if focused.

https://cs.chromium.org/chromium/src/ui/file_manager/gallery/css/gallery.css?...

[fukino, 2016/10/06 14:34:30]

The background of this button is darker, so (0, 0, 0) is almost invisible.
I picked this value from Material Design button spec for dark background.
https://material.google.com/components/buttons.html#buttons-raised-buttons

(BTW, it says "Focused:12% #000000 shade over the color", but I suppose the
correct spec is "Focused:12% #FFFFFF shade over the color".)

+}
+
 .edit-modal button.edit-toolbar .icon {
   display: none;
 }
 
 .edit-modal button.edit-toolbar .label {
   display: block;
 }
 
 /* Action buttons */
 .edit-modal-wrapper .action-buttons button {
@@ skipping 564 matching lines @@
 .thumbnail-view .animation-thumbnail {
   background-repeat: no-repeat;
   background-size: cover;
   display: none;
   position: absolute;
 }
 
 .thumbnail-view .animation-thumbnail.animating {
   display: block;
 }