Supervised user "learn more" page: Fix css flex usage.

chrome/browser/resources/options/supervised_user_learn_more.css

 /* 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. */
 
 #supervised-user-learn-more {
+  display: flex;
+  flex-direction: column;
   width: 722px;
 }
 
 #supervised-user-learn-more-title {
   padding: 20px;
 }
 
 #supervised-user-learn-more-text {
   padding: 0 20px 0 20px;
   white-space: pre-wrap;
   word-wrap: break-word;
 }
 
 @media only screen and (max-height:500px) {
 
 /* Omit the image on very small screens. */
 #supervised-user-learn-more-image {
   display: none;
 }
 
 } /* @media only screen and (max-height:500px) */
 
 #supervised-user-learn-more-image {
   -webkit-border-radius: 3px 3px 0 0;
   background-image: -webkit-image-set(
       url(../../../../ui/resources/default_100_percent/supervised_illustration_start.png) 1x,
       url(../../../../ui/resources/default_200_percent/supervised_illustration_start.png) 2x);
   background-position: center;
-  flex: 1;
   height: 344px;
 }
 
 #supervised-user-learn-more-action-area {
   padding: 20px;
 }