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

ui/file_manager/audio_player/elements/audio_player.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.
 
 Polymer({
   is: 'audio-player',
 
   properties: {
     /**
      * Flag whether the audio is playing or paused. True if playing, or false
@@ skipping 119 matching lines @@
    * @param {number} newValue new value.
    * @param {number} oldValue old value.
    */
   currentTrackIndexChanged: function(newValue, oldValue) {
     var currentTrackUrl = '';
 
     if (oldValue != newValue) {
       var currentTrack = this.$.trackList.getCurrentTrack();
       if(currentTrack && currentTrack != this.$.trackInfo.track){
         this.$.trackInfo.track = currentTrack;
+        this.$.trackInfo.artworkAvailable = !!currentTrack.artworkUrl;
       }
       if (currentTrack && currentTrack.url != this.$.audio.src) {
         this.$.audio.src = currentTrack.url;
         currentTrackUrl = this.$.audio.src;
         if (this.playing)
           this.$.audio.play();
       }
     }
 
     // The attributes may be being watched, so we change it at the last.
@@ skipping 220 matching lines @@
     this.$.trackList.notifyPath('tracks.' + index + '.title',
         this.tracks[index].title);
     this.$.trackList.notifyPath('tracks.' + index + '.artist',
         this.tracks[index].artist);
 
     if (this.$.trackInfo.track &&
         this.$.trackInfo.track.url === this.tracks[index].url){
       this.$.trackInfo.notifyPath('track.title', this.tracks[index].title);
       this.$.trackInfo.notifyPath('track.artist', this.tracks[index].artist);
       var artworkUrl = this.tracks[index].artworkUrl;
-      if (!artworkUrl || artworkUrl !== '')
+      if (!artworkUrl && artworkUrl !== '') {
         this.$.trackInfo.notifyPath('track.artworkUrl',
             this.tracks[index].artworkUrl);
-      else
+        this.$.trackInfo.artworkAvailable = true;
+      } else {
         this.$.trackInfo.notifyPath('track.artworkUrl', undefined);
+        this.$.trackInfo.artworkAvailable = false;
+      }
     }
   },
 
   /**
    * Invoked when the audio player is being unloaded.
    */
   onPageUnload: function() {
     this.$.audio.src = '';  // Hack to prevent crashing.
   },
 
@@ skipping 38 matching lines @@
 
   /**
    * Computes volume value for audio element. (should be in [0.0, 1.0])
    * @param {number} volume Volume which is set in the UI. ([0, 100])
    * @return {number}
    */
   computeAudioVolume_: function(volume) {
     return volume / 100;
   }
 });