Intorduce a separate preference for 'proxy server mode'

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

Index: chrome/test/data/extensions/api_test/proxy/system/test.js
diff --git a/chrome/test/data/extensions/api_test/proxy/system/test.js b/chrome/test/data/extensions/api_test/proxy/system/test.js
new file mode 100644
index 0000000000000000000000000000000000000000..1910ca8edda1bac2c01a77ce6c41da488f1ad428
--- /dev/null
+++ b/chrome/test/data/extensions/api_test/proxy/system/test.js
@@ -0,0 +1,14 @@
+// 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.ProxySystem
+
+chrome.test.runTests([
+  function setSystemProxy() {
+    var config = { mode: "system" };
+    chrome.experimental.proxy.useCustomProxySettings(config);
+    chrome.test.succeed();
+  }
+]);