AudioPlayer.app: Use HTML Imports to define custom elements.

ui/file_manager/audio_player/elements/track_list.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/polymer/polymer.html">
+
+<polymer-element name="track-list" attributes="tracks">
+  <template>
+    <link rel="stylesheet" href="track_list.css">
+
+    <template id="tracks" repeat="{{track, index in tracks}}">
+      <div class="track" active?="{{track.active}}" index="{{index}}" on-click="{{trackClicked}}">
+        <div class="data">
+          <div class="data-title">{{track.title}}</div>
+          <div class="data-artist">{{track.artist}}</div>
+        </div>
+      </div>
+    </template>
+  </template>
+  <script src="track_list.js"></script>
+</polymer-element>