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

ui/file_manager/externs/es7_workaround.js

Index: ui/file_manager/externs/es7_workaround.js
diff --git a/ui/file_manager/externs/es7_workaround.js b/ui/file_manager/externs/es7_workaround.js
index 4d76e88e6f7eeb4ad2c6c07c6863993589a05629..e1275084067096d357b6eb480106929950e7a9e9 100644
--- a/ui/file_manager/externs/es7_workaround.js
+++ b/ui/file_manager/externs/es7_workaround.js
@@ -10,19 +10,6 @@
 */
 
 /**
- * @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
  */