Update CSS to delay change in visibility so it matches the overlay

chrome/browser/resources/print_preview/previewarea/preview_area.css

 /* Copyright (c) 2012 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. */
 
 #preview-area.preview-area {
   -webkit-box-flex: 1;
   -webkit-user-select: none;
   background-color: #e6e6e6;
   overflow: hidden;
   position: relative;
@@ skipping 62 matching lines @@
 
 #preview-area .preview-area-no-plugin-action-area {
   margin-top: 12px;
 }
 
 #preview-area .preview-area-open-system-dialog-button-throbber {
   vertical-align: middle;
 }
 
 .preview-area-overlayed {
+  /**
+   * The 100 ms delay here is intended to match that of
+   * #preview-area .preview-area-overlay-layer.invisible
+   * This is important because without this delay there will be a noticeable
+   * flicker whenever the overlay is displayed for a short period of time.
+   */
+  transition: visibility 100ms linear 100ms;

[Dan Beam, 2014/11/04 19:15:13]

i'm amazed visibility is interpolated.
https://developer.mozilla.org/en-US/docs/Web/CSS/visibility#Interpolation

lgtm as long as this is cancelled when new previews finish loading/rendering.

   visibility: hidden;
 }