[quickview] Use arrow functions for delete confirm dialog callbacks

Tbr: alex
Bug: 803259
Change-Id: Ib0872eea67e4d28fc01056c946b48bde04eb5159
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035635Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737967}

ui/file_manager/file_manager/foreground/js/quick_view_controller.js

@@ -289,11 +289,11 @@ class QuickViewController {
         event.stopPropagation();
       });
 
-      this.deleteConfirmDialog_.showModalElement = function() {
+      this.deleteConfirmDialog_.showModalElement = () => {
         dialogElement.showModal();
       };
 
-      this.deleteConfirmDialog_.doneCallback = function() {
+      this.deleteConfirmDialog_.doneCallback = () => {
         dialogElement.close();
       };
     }