AudioPlayer: Fix the way to find a track element with given index.

ui/file_manager/audio_player/elements/track_list.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.
 
 /**
  * @typedef {?{
  *   url: string,
  *   title: string,
  *   artist: string,
  *   artwork: Object,
@@ skipping 173 matching lines @@
     onWindowResize_: function() {
       this.ensureTrackInViewport_(this.currentTrackIndex);
     },
 
     /**
      * Scrolls the track list to ensure the given track in the viewport.
      * @param {number} trackIndex The index of the track to be in the viewport.
      * @private
      */
     ensureTrackInViewport_: function(trackIndex) {
-      var trackSelector = '::shadow .track[index="' + trackIndex + '"]';
-      var trackElement = this.querySelector(trackSelector);
+      var trackElement = this.$$('.track[index="' + trackIndex + '"]');
       if (trackElement) {
         var viewTop = this.scrollTop;
         var viewHeight = this.clientHeight;
         var elementTop = trackElement.offsetTop;
         var elementHeight = trackElement.offsetHeight;
 
         if (elementTop < viewTop) {
           // Adjust the tops.
           this.scrollTop = elementTop;
         } else if (elementTop + elementHeight <= viewTop + viewHeight) {
@@ skipping 122 matching lines @@
 
       var newTrackIndex = this.playOrder[newPlayOrder];
       console.assert(
           (0 <= newTrackIndex && newTrackIndex < this.tracks.length),
           'Insufficient TrackList.playOrder. New Play Order: ' + newPlayOrder);
 
       return newTrackIndex;
     },
   });
 })();  // Anonymous closure