Files.app: Make ProgressCenterItemGroup class to manage the states of items.

chrome/browser/resources/file_manager/common/js/progress_center_common.js

 // Copyright 2013 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.
 
 'use strict';
 
 /**
  * Event of the ProgressCenter class.
  * @enum {string}
  * @const
@@ skipping 137 matching lines @@
   /**
    * Whether the item can be canceled or not.
    * @return {boolean} True if the item can be canceled.
    */
   get cancelable() {
     return !!(this.state == ProgressItemState.PROGRESSING &&
               this.cancelCallback &&
               !this.summarized);
   }
 };
+
+/**
+ * Clones the item.
+ * @return {ProgressCenterItem} New item having the same properties with this.
+ */
+ProgressCenterItem.prototype.clone = function() {
+  var newItem = new ProgressCenterItem();
+  newItem.id = this.id;
+  newItem.state = this.state;
+  newItem.message = this.message;
+  newItem.progressMax = this.progressMax;
+  newItem.progressValue = this.progressValue;
+  newItem.type = this.type;
+  newItem.summarized = this.summarized;
+  newItem.cancelCallback = this.cancelCallback;
+  return newItem;
+};