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

BUG=429652

Review URL: https://codereview.chromium.org/700463005

Cr-Commit-Position: refs/heads/master@{#302658}

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

@@ -80,5 +80,12 @@
 }
 
 .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;
   visibility: hidden;
 }