Fix overlay.css so close button is on top of the page.

chrome/browser/resources/shared/css/overlay.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. */
 
 /* The shield that overlays the background. */
 .overlay {
   -webkit-box-align: center;
   -webkit-box-orient: vertical;
   -webkit-box-pack: center;
   -webkit-transition: 200ms opacity;
@@ skipping 20 matching lines @@
   -webkit-border-radius: 3px;
   -webkit-box-orient: vertical;
   -webkit-transition: 200ms -webkit-transform;
   background: white;
   box-shadow: 0 4px 23px 5px rgba(0, 0, 0, 0.2), 0 2px 6px rgba(0,0,0,0.15);
   color: #333;
   display: -webkit-box;
   min-width: 400px;
   padding: 0;
   position: relative;
+  z-index: 0;
 }
 
 /* If the options page is loading don't do the transition. */
 .loading .overlay,
 .loading .overlay .page {
   -webkit-transition-duration: 0 !important;
 }
 
 /* keyframes used to pulse the overlay */
 @-webkit-keyframes pulse {
@@ skipping 20 matching lines @@
 
 .overlay .page > .close-button {
   background-image: url('chrome://theme/IDR_WEB_UI_CLOSE');
   background-position: center;
   background-repeat: no-repeat;
   height: 14px;
   position: absolute;
   right: 7px;
   top: 7px;
   width: 14px;
+  z-index: 1;
 }
 
 html[dir='rtl'] .overlay .page > .close-button {
   left: 10px;
   right: auto;
 }
 
 .overlay .page > .close-button:hover {
   background-image: url('chrome://theme/IDR_WEB_UI_CLOSE_HOVER');
 }
@@ skipping 63 matching lines @@
 .overlay .page .content-area::-webkit-scrollbar-thumb {
   background-color: rgba(0, 0, 0, 0.2);
   border: 2px solid white;
   border-radius: 8px;
 }
 
 .overlay .page .content-area::-webkit-scrollbar-thumb:hover {
   background-color: rgba(0, 0, 0, 0.5);
 }
 </if>