Don't allow elevation for CHROME_OS.

chrome/browser/resources/managed_mode_block_interstitial.js

 // Copyright (c) 2012 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.
 
 function sendCommand(cmd) {
   window.domAutomationController.setAutomationId(1);
   window.domAutomationController.send(cmd);
 }
 
 function initialize() {
-  $('bypass-block-button').onclick = function(event) {
-    sendCommand('preview');
+  if (!cr.isChromeOS) {
+    $('bypass-block-button').onclick = function(event) {
+      sendCommand('preview');
+    }
   }
   $('back-button').onclick = function(event) {
     sendCommand('back');
   };
   $('content-packs-section-button').onclick = function(event) {
     sendCommand('ntp');
   };
 }
 
 document.addEventListener('DOMContentLoaded', initialize);