Use PageManager in About page

ui/webui/resources/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;
   /* TODO(dbeam): remove perspective when http://crbug.com/374970 is fixed. */
@@ skipping 161 matching lines @@
 
 .gray-bottom-bar {
   background-color: #f5f5f5;
   border-color: #e7e7e7;
   border-top-style: solid;
   border-width: 1px;
   color: #888;
   display: -webkit-box;
   padding: 14px 17px;
 }
+
+/* Prevent the page underneath the overlay from scrolling. */
+.frozen {
+  position: fixed;
+}
+
+#overlay-container-1 {
+  z-index: 11;
+}
+#overlay-container-2 {
+  z-index: 12;
+}
+
+.transparent {
+  opacity: 0;
+}