Introduce a field trial for Instant.

chrome/browser/resources/options/instant_confirm_overlay.js

 // Copyright (c) 2011 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.
 
 cr.define('options', function() {
   var OptionsPage = options.OptionsPage;
 
   function InstantConfirmOverlay() {
     OptionsPage.call(this, 'instantConfirm',
                      templateData.instantConfirmTitle,
                      'instantConfirmOverlay');
   };
 
   cr.addSingletonGetter(InstantConfirmOverlay);
 
   InstantConfirmOverlay.prototype = {
     // Inherit from OptionsPage.
     __proto__: OptionsPage.prototype,
 
     initializePage: function() {
       OptionsPage.prototype.initializePage.call(this);
 
       $('instantConfirmCancel').onclick = function() {
         OptionsPage.closeOverlay();
+        $('instantEnabledCheckbox').checked = false;
       };
+
       $('instantConfirmOk').onclick = function() {
         OptionsPage.closeOverlay();
-        Preferences.setBooleanPref('instant.confirm_dialog_shown', true);
-        var instantEnabledCheckbox = $('instantEnableCheckbox');
-        Preferences.setBooleanPref(instantEnableCheckbox.pref, true,
-                                   instantEnableCheckbox.metric);
+        chrome.send('enableInstant');
       };
     },
   };
 
   // Export
   return {
     InstantConfirmOverlay: InstantConfirmOverlay
   };
 });