Implement "expand/collapse artwork" button in audio player.

ui/file_manager/audio_player/css/audio_player.css

 /* Copyright 2013 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. */
 
 html {
   height: 100%;
 }
 
 body {
   -webkit-user-select: none;
@@ skipping 10 matching lines @@
   border-bottom: 1px solid #ddd;
   height: 32px;
 }
 
 div.audio-player {
   border-top: 4px solid white;
   color: #3d3d3d;
   cursor: default;
   flex: 1 1 auto;
   font-size: 10pt;
+  box-sizing: border-box;

[fukino, 2016/02/02 08:33:33]

nit: sort properties.

   height: 100%;
   position: relative;
 }
 
 .audio-player:not(.collapsed):not(.single-track) > .title-button {
   background-color: #1f1f1f;
 }
 
 /* Customized scrollbar for the playlist. */
 
@@ skipping 27 matching lines @@
   border-right: 0 solid transparent;
   border-top: 0 solid transparent;
 }
 
 ::-webkit-scrollbar-track:vertical {
   background-clip: padding-box;
   background-color: rgba(255, 255, 255, 0.5);
   border-left: 5px solid transparent;
   border-right: 0 solid transparent;
 }