Update CSS for supervised user block interstitial to be more friendly to narrow screens.

chrome/browser/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(230, 230, 230);
   font-family: Helvetica, Arial, sans-serif;
   font-size: 10pt;
   margin: 50px 40px 20px 40px;
   text-align: center;
 }
@@ skipping 34 matching lines @@
    * the resource manually. */
   -webkit-user-select: none;
   content: -webkit-image-set(
       url('../../app/theme/default_100_percent/common/error_managed_mode_blocked_page.png') 1x,
       url('../../app/theme/default_200_percent/common/error_managed_mode_blocked_page.png') 2x);
   margin-bottom: 10px;
   margin-top: 10px;
 }
 
 #content-top {
-  margin: 20px 20px 20px 25px;
+  margin: 20px 25px;
 }
 
 button {
   -webkit-user-select: none;
   background-image: linear-gradient(rgb(246, 246, 246) 5%,
                                     rgb(239, 239, 239) 50%,
                                     rgb(221, 221, 221));
   border: 1px solid rgb(209, 209, 211);
   border-bottom: 1px solid rgb(193, 193, 195);
   border-radius: 2px;
   box-shadow: inset 0 1px 0 rgb(255, 255, 255);
   color: rgb(102, 102, 102);
   font-weight: bold;
-  margin: 0 5px;
+  margin: 5px 0;
   padding: 8px 13px;
   text-shadow: 0 1px 0 rgba(255, 255, 255, 0.8);
 }
 
 #back-button {
   background-image: linear-gradient(rgb(84, 153, 244) 5%,
                                     rgb(82, 148, 242) 50%,
                                     rgb(75, 133, 241));
   border: 1px solid rgb(81, 135, 223);
   border-bottom: 1px solid rgb(56, 112, 207);
   box-shadow: inset 0 1px 0 rgb(95, 168, 247);
   color: rgb(255, 255, 255);
   text-shadow: 0 1px 0 rgba(0, 0, 0, 0.2);
-  width: 100px;
 }
+
+@media (min-width: 376px) {
+  #back-button {
+    width: 100px;
+  }
+}
+
+@media (max-width: 375px) {
+  button {
+    width: 100%;
+  }
+}