Add custom interstitial for captive portals.

chrome/browser/resources/security_warnings/interstitial_v2.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. */
 
 a {
   color: #585858;
 }
 
 .bad-clock .icon {
   background-image: -webkit-image-set(
@@ skipping 156 matching lines @@
   color: #696969;
   font-size: .875em;
 }
 
 .ssl .icon {
   background-image: -webkit-image-set(
       url(images/1x/brokenssl_red.png) 1x,
       url(images/2x/brokenssl_red.png) 2x);
 }
 
+.captive-portal .icon {
+  background-image: -webkit-image-set(
+      url(../../../app/theme/default_100_percent/common/error_network_offline.png) 1x,
+      url(../../../app/theme/default_200_percent/common/error_network_offline.png) 2x);
+}
+
 .styled-checkbox {
   float: left;
   height: 16px;
   margin-top: .36em;
   position: relative;
   width: 16px;
 }
 
 [dir='rtl'] .styled-checkbox {
   float: right;
@@ skipping 74 matching lines @@
   }
 
   .nav-wrapper {
     margin-top: 30px;
   }
 
   .small-link {
     font-size: 1em;
   }
 }