Introduce a separate preference for 'proxy server mode'

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

Index: chrome/test/data/extensions/api_test/proxy/single/test.js
diff --git a/chrome/test/data/extensions/api_test/proxy/single/test.js b/chrome/test/data/extensions/api_test/proxy/single/test.js
index a20fb316cbc64d1798779872049863fbe1a648ea..9cc8502b24be2b886d91c759f59f0a881d893722 100644
--- a/chrome/test/data/extensions/api_test/proxy/single/test.js
+++ b/chrome/test/data/extensions/api_test/proxy/single/test.js
@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 // proxy api test
-// browser_tests.exe --gtest_filter=ExtensionApiTest.ProxyManualSingle
+// browser_tests.exe --gtest_filter=ExtensionApiTest.ProxyFixedSingle
 
 chrome.test.runTests([
   function setSingleProxy() {
@@ -29,7 +29,7 @@ chrome.test.runTests([
       socksProxy: socksProxy,
     };
 
-    var config = { rules: rules };
+    var config = { rules: rules, mode: "fixed_servers" };
     chrome.experimental.proxy.useCustomProxySettings(config);
     chrome.test.succeed();
   }