Use Object.preventExtensions instead of Object.seal.

* Object.seal use the more memory than Object.preventExtensions.

BUG=402708

Review URL: https://codereview.chromium.org/551353002

Cr-Commit-Position: refs/heads/master@{#293922}

ui/file_manager/file_manager/common/js/progress_center_common.js

@@ -115,7 +115,9 @@ var ProgressCenterItem = function() {
    */
   this.cancelCallback = null;
 
-  Object.seal(this);
+  // This object is instantiated many time. Object.seal use more memory
+  // than Object.preventExtensions (crbug.com/412307)
+  Object.preventExtensions(this);
 };
 
 ProgressCenterItem.prototype = {