[MD Bookmarks] Refactor window timer mocking.

chrome/browser/resources/md_bookmarks/timer_proxy.js

+// Copyright 2017 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 A class that mediates window timer calls to support mockability
+ * in tests.
+ */
+
+cr.define('bookmarks', function() {
+  /** @constructor */
+  function TimerProxy() {}
+
+  TimerProxy.prototype = {
+    /**
+     * @param {Function|string} fn
+     * @param {number=} delay
+     * @return {number}
+     */
+    setTimeout: function(fn, delay) {
+      return window.setTimeout(fn, delay);
+    },
+
+    /** @param {number|undefined?} id */
+    clearTimeout: function(id) {
+      window.clearTimeout(id);
+    },
+  };
+
+  return {
+    TimerProxy: TimerProxy,
+  };
+});