Enable (optional) blocking of webrequests in case a PAC script cannot be fetched or is invalid.

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

 // Copyright (c) 2011 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=ProxySettingsApiTest.ProxyPacData
 
 function expect(expected, message) {
   return chrome.test.callbackPass(function(value) {
     chrome.test.assertEq(expected, value, message);
   });
 }
 
 chrome.test.runTests([
   function setAutoSettings() {
     var pacScriptObject = {
       data: "function FindProxyForURL(url, host) {\n" +
             "  if (host == 'foobar.com')\n" +
             "    return 'PROXY blackhole:80';\n" +
             "  return 'DIRECT';\n" +
-            "}"
+            "}",
+      mandatory: false
     };
     var config = {
       mode: "pac_script",
       pacScript: pacScriptObject
     };
     chrome.experimental.proxy.settings.set(
         {'value': config},
         chrome.test.callbackPass());
     chrome.experimental.proxy.settings.get(
         {'incognito': false},
         expect({ 'value': config,
                  'levelOfControl': "ControlledByThisExtension" },
                "invalid proxy settings"));
   }
 ]);