AudioPlayer: Start playing a track when play icon on the track list is clicked.

ui/file_manager/audio_player/elements/track_list.css

 /* Copyright 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. */
 
 :host {
   align-items: center;
   background: rgb(245, 245, 245);
   bottom: 72px;  /* Room for the controls bar. */
   color: rgb(51, 51, 51);
   cursor: default;
@@ skipping 28 matching lines @@
   background-color: rgba(0, 0, 0, 0.08);
 }
 
 /* Track icon. */
 .track .icon {
   background-position: center;
   background-repeat: no-repeat;
   flex: none;
   height: 32px;
   margin: 8px;
+  pointer-events: none;
   width: 32px;
 }
 
 .track:hover .icon {
   background-image: -webkit-image-set(
       url(../assets/100/playlist_play.png) 1x,
       url(../assets/200/playlist_play.png) 2x);
+  pointer-events: auto;
 }
 
 .track[active] .icon {
   background-image: -webkit-image-set(
       url(../assets/100/playlist_now_playing.png) 1x,
       url(../assets/200/playlist_now_playing.png) 2x);
 }
 
 /* Track data. */
 
@@ skipping 16 matching lines @@
 .track .data .data-title {
   color: rgb(51, 51, 51);
   font-size: 13px;
   font-weight: 500;
 }
 
 .track .data .data-artist {
   color: rgb(100, 100, 100);
   font-size: 12px;
 }