Introduce a separate preference for 'proxy server mode'

chrome/test/data/extensions/api_test/proxy/individual/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.ProxyManualIndividual
+// browser_tests.exe --gtest_filter=ExtensionApiTest.ProxyFixedIndividual
 
 chrome.test.runTests([
   function setIndividualProxies() {
     var httpProxy = {
       host: "1.1.1.1"
     };
     var httpsProxy = {
       scheme: "socks",
       host: "2.2.2.2"
     };
     var ftpProxy = {
       host: "3.3.3.3",
       port: 9000
     };
     var socksProxy = {
       scheme: "socks4",
       host: "4.4.4.4",
       port: 9090
     };
 
     var rules = {
       proxyForHttp: httpProxy,
       proxyForHttps: httpsProxy,
       proxyForFtp: ftpProxy,
       socksProxy: socksProxy,
     };
 
-    var config = { rules: rules };
+    var config = { rules: rules, mode: "fixed_servers" };
     chrome.experimental.proxy.useCustomProxySettings(config);
     chrome.test.succeed();
   }
 ]);