Support persistent incognito preferences

chrome/test/data/extensions/api_test/proxy/individual_incognito_only/test.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.
 
 // proxy api test
 // browser_tests.exe
 //     --gtest_filter=ProxySettingsApiTest.ProxyFixedIndividualIncognitoOnly
 
 chrome.test.runTests([
   // Verify that execution has started to make sure flaky timeouts are not
@@ skipping 21 matching lines @@
 
     var rules = {
       proxyForHttp: httpProxy,
       proxyForHttps: httpsProxy,
       proxyForFtp: ftpProxy,
       fallbackProxy: fallbackProxy,
     };
 
     var config = { rules: rules, mode: "fixed_servers" };
     chrome.proxy.settings.set(
-        {'value': config, 'incognito': true},
+        {'value': config, 'scope': 'incognito_persistent'},
         chrome.test.callbackPass());
   }
 ]);