Supervised User interstitial: Update strings to be more child-friendly

components/supervised_user_error_page/resources/supervised_user_block_interstitial.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. */
 body {
   background-color: rgb(247, 247, 247);
   font-size: 10pt;
   margin: 150px 60px 0 30px;
 }
 
 #main-frame-blocked {
   margin: auto;
   max-width: 600px;
   min-width: 200px;
 }
 
 h1 {
   font-size: 1.8em;
   font-weight: normal;
   margin: 5px 0 25px 0;
 }
 
+#block-page-header {
+  font-weight: bold;
+  margin-bottom: 20px;

[Marc Treib, 2016/11/08 16:33:44]

This is totally made up :)  Probably Rachel should take a look. I'll make
screenshots.

+}
+
 .avatar-img {
   -webkit-user-select: none;
   border: 3px solid rgb(251, 251, 251);
   border-radius: 50%;
   content: -webkit-image-set(
       url(default_100_percent/logo_avatar_circle_blue_color.png) 1x,
       url(default_200_percent/logo_avatar_circle_blue_color.png) 2x);
   margin-bottom: 5px;
   margin-right: 15px;
   margin-top: 5px;
@@ skipping 107 matching lines @@
 
   #main-frame-blocked {
     display: flex;
     flex-flow: column;
   }
 
   #feedback {
     margin-top: 35px;
   }
 }