AudioPlayer: Update volume controls.

ui/file_manager/audio_player/elements/audio_player.html

 <!--
   -- Copyright 2015 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.
   -->
 
 <link rel="import" href="chrome://resources/polymer/v1_0/polymer/polymer.html">
 <link rel="import" href="track_list.html">
 <link rel="import" href="control_panel.html">
 
 <dom-module id="audio-player">
   <link rel="import" type="css" href="audio_player.css">
   <template>
     <track-list id="trackList"
         expanded$="[[expanded]]"
         shuffle="[[shuffle]]"
         current-track-index="{{currentTrackIndex}}"
         on-replay="onReplayCurrentTrack"></track-list>
     <control-panel id="audioController"
         playing="{{playing}}"
         time="{{time}}"
         duration="[[duration]]"
         shuffle="{{shuffle}}"
         repeat="{{repeat}}"
         volume="{{volume}}"
         expanded="{{expanded}}"
+        volume-slider-shown="{{volumeSliderShown}}"
         on-next-clicked="onControllerNextClicked"
         on-previous-clicked="onControllerPreviousClicked"></control-panel>
     <audio id="audio"
         volume="[[computeAudioVolume_(volume)]]"></audio>
   </template>
 </dom-module>
 
 <script src="audio_player.js"></script>