[quickview] Style the delete-confirm-dialog

The <dialog> element added in CL:2033012 is too dark. We should restyle
it to lighten it up.

Use transparent to allow the cr.ui.dialog background-color: rgba(...)
to control the apparent lightness.

No change in behavior, no new tests.

Bug: 803259
Change-Id: I9a7a4b89419a2cc55d2aa249d17464d8bd523347
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035822
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738088}

ui/file_manager/file_manager/foreground/elements/files_quick_view.css

@@ -32,6 +32,10 @@ dialog::backdrop {
   opacity: 0.6;
 }
 
+dialog#delete-confirm-dialog::backdrop {
+  background-color: transparent;
+}
+
 .thumbnail {
   flex: none;
   height: 100%;

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

@@ -278,7 +278,6 @@ class QuickViewController {
 
     // Create a delete confirm dialog if needed.
     if (!this.deleteConfirmDialog_) {
-      // TODO(crbug.com/803259): style this element to make it lighter.
       const dialogElement = document.createElement('dialog');
       this.quickView_.shadowRoot.appendChild(dialogElement);
       dialogElement.id = 'delete-confirm-dialog';