AudioPlayer: Update volume controls.

ui/file_manager/audio_player/elements/audio_player.js

Index: ui/file_manager/audio_player/elements/audio_player.js
diff --git a/ui/file_manager/audio_player/elements/audio_player.js b/ui/file_manager/audio_player/elements/audio_player.js
index 31a48010cc1b9f4225f624ecc0a911aeb58f6b45..40bd55e721b463859887f0c84a3b4baa81ed47f5 100644
--- a/ui/file_manager/audio_player/elements/audio_player.js
+++ b/ui/file_manager/audio_player/elements/audio_player.js
@@ -57,6 +57,14 @@ Polymer({
     },
 
     /**
+     * Whether the volume slider is shown.
+     */
+    volumeSliderShown: {
+      type: Boolean,
+      observer: 'volumeSliderShownChanged'
+    },
+
+    /**
      * Track index of the current track.
      */
     currentTrackIndex: {
@@ -135,6 +143,14 @@ Polymer({
   },
 
   /**
+   * Handles change event for volumeSliderShown state.
+   */
+  volumeSliderShownChanged: function(volumeSliderShown) {
+    if (this.model)
+      this.model.volumeSliderShown = volumeSliderShown;
+  },
+
+  /**
    * Initializes an element. This method is called automatically when the
    * element is ready.
    */