Intorduce a separate preference for 'proxy server mode'

chrome/test/data/extensions/api_test/proxy/single/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.ProxyManualSingle
 
 chrome.test.runTests([
   function setSingleProxy() {
     var oneProxy = {
@@ skipping 11 matching lines @@
     var socksProxy = {
       host: "9.9.9.9"
     };
 
     var rules = {
       singleProxy: oneProxy,
       proxyForHttp: httpProxy,
       socksProxy: socksProxy,
     };
 
-    var config = { rules: rules };
+    var config = { rules: rules, mode: "manual" };
     chrome.experimental.proxy.useCustomProxySettings(config);
     chrome.test.succeed();
   }
 ]);