Treat policy-disallowed sync xhr as a network error

Bug: 661283

Change-Id: I311cef40e03a264ca9db601842477d502e7cf12d
Reviewed-on: https://chromium-review.googlesource.com/804057
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529775}

xhr/xmlhttprequest-sync-default-feature-policy.sub.html

@@ -11,8 +11,7 @@
   run_all_fp_tests_allow_all(
       'http://{{domains[www]}}:{{ports[http][0]}}',
       'sync-xhr',
-      'InvalidAccessError: Failed to execute \'open\' on \'XMLHttpRequest\': ' +
-      'Synchronous requests are disabled by Feature Policy.',
+      'NetworkError',
       () => {
         return new Promise((resolve, reject) => {
           try {