Update directory tree's scrollbar when the progress center mutates.

ui/file_manager/file_manager/foreground/js/ui/progress_center_panel.js

Index: ui/file_manager/file_manager/foreground/js/ui/progress_center_panel.js
diff --git a/ui/file_manager/file_manager/foreground/js/ui/progress_center_panel.js b/ui/file_manager/file_manager/foreground/js/ui/progress_center_panel.js
index 2f33d398c22b514c2149abb8d4e5d990d87f816a..3d65e9ed041d2f90aa4c903bbe9014e54a60eef4 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/progress_center_panel.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/progress_center_panel.js
@@ -280,6 +280,16 @@ ProgressCenterPanel.getToggleAnimation_ = function(document) {
  */
 ProgressCenterPanel.RESET_DELAY_TIME_MS_ = 5000;
 
+ProgressCenterPanel.prototype = {
+  /**
+   * Root element of the progress center.
+   * @type {HTMLElement}
+   */
+  get element() {
+    return this.element_;
+  }
+};
+
 /**
  * Updates an item to the progress center panel.
  * @param {!ProgressCenterItem} item Item including new contents.