Introduce a separate preference for 'proxy server mode'

chrome/test/data/extensions/api_test/proxy/auto/test.js

 // Copyright (c) 2010 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=ExtensionApiTest.ProxyAutoSettings
 
 chrome.test.runTests([
   function setAutoSettings() {
-    var pacScriptObject = {
-      url: "http://wpad/windows.pac"
-    };
     var config = {
-      autoDetect: true,
-      pacScript: pacScriptObject
+      mode: "auto_detect",
     };
     chrome.experimental.proxy.useCustomProxySettings(config);
     chrome.test.succeed();
   }
 ]);