Introduce a separate preference for 'proxy server mode'

chrome/test/data/extensions/api_test/proxy/pac/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.ProxyPacScript
+
+chrome.test.runTests([
+  function setAutoSettings() {
+    var pacScriptObject = {
+      url: "http://wpad/windows.pac"
+    };
+    var config = {
+      mode: "pac_script",
+      pacScript: pacScriptObject
+    };
+    chrome.experimental.proxy.useCustomProxySettings(config);
+    chrome.test.succeed();
+  }
+]);