AudioPlayer: Update volume controls.

ui/file_manager/audio_player/js/audio_player_model.js

 // 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.
 
 /**
  * The model class for audio player.
  * @constructor
  */
 function AudioPlayerModel() {}
 
 AudioPlayerModel.prototype.initialize = function(callback) {
   'use strict';
 
   /**
    * List of values to be stored into the model.
    * @type {!Object<*>}
    * @const
    */
   var VALUES =
     /**
      * They will be used as properties of AudioPlayerModel.
      * @lends {AudioPlayerModel.prototype}
      */
     {
       shuffle: false,
       repeat: false,
       volume: 100,
       expanded: false,
+      volumeSliderShown: false
     };
 
   /**
    * Prefix of the stored values in the storage.
    * @type {string}
    */
   var STORAGE_PREFIX = 'audioplayer-';
 
   /**
    * Save the values in the model into the storage.
@@ skipping 38 matching lines @@
   // Restores the values from the storage
   var target = this;
   loadModel(target, function() {
     // Installs observer to watch changes of the values.
     Object.observe(target, function(changes) {
       saveModel(target);
     });
     callback();
   });
 }