Use PageManager in About page

chrome/browser/resources/help/help_content.css

-/* Copyright (c) 2012 The Chromium Authors. All rights reserved.
+/* Copyright 2014 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.
  */
 
-body {
-  -webkit-user-select: text;
-}
-
-#overlay .page:not(.showing) {
-  display: none;
-}
-
 #about-container {
-  -webkit-box-align: center;
-  display: -webkit-box;
+  align-items: center;
+  display: flex;
   margin-top: 10px;
 }
 
 #product-description {
   -webkit-margin-start: 10px;
 }
 
 #version-container {
   margin-top: 30px;
 }
@@ skipping 21 matching lines @@
 
 #help-container {
   margin-top: 16px;
 }
 
 #product-container {
   line-height: 1.8em;
   margin-top: 200px;
 }
 
+.overlay #product-container {
+  margin-top: 40px;
+}
+
 .help-page-icon,
 .help-page-icon-large {
   background-repeat: no-repeat;
   display: inline-block;
   margin-right: 4px;
   vertical-align: top;
 }
 
 .help-page-icon {
   height: 17px;
@@ skipping 52 matching lines @@
 }
 
 #channel-change-disallowed-icon {
   -webkit-margin-start: 4px;
   background-image: url('chrome://theme/IDR_CONTROLLED_SETTING_MANDATORY');
   background-size: 17px;
   vertical-align: middle;
 }
 
 .channel-change-error-bubble {
-  display: -webkit-box;
+  display: flex;
 }
 
 .channel-change-error-icon {
   -webkit-margin-start: 4px;
   background-image: url('chrome://theme/IDR_CONTROLLED_SETTING_MANDATORY');
   background-size: 22px;
   vertical-align: top;
 }
 
 .channel-change-error-text {
   -webkit-margin-start: 4px;
   display: block;
   vertical-align: top;
   width: 240px;
 }