AudioPlayer: Stop using Object.observe() and Array.observe().

ui/file_manager/externs/es7_workaround.js

-// 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.
-
-/**
-* @fileoverview Partial definitions for ECMAScript 7. To compile Files.app, some
-*     definitions are defined in this file. They should be removed once ES7
-*     definitions are ready in closure compiler by default.
-* @externs
-*/
-
-/**
- * @param {!Object} obj
- * @param {function(!Array<!Object>)} callback
- * @param {!Array<string>=} acceptList
- */
-Object.observe = function(obj, callback, acceptList) {};
-
-/**
- * @param {!Object} obj
- * @param {function(!Array<!Object>)} callback
- */
-Object.unobserve = function(obj, callback) {};
-
-/**
- * @param {!Array} arr
- * @param {function(!Array<!Object>)} callback
- */
-Array.observe = function(arr, callback) {};
-
-/**
- * @param {!Array} arr
- * @param {function(!Array<!Object>)} callback
- */
-Array.unobserve = function(arr, callback) {};