Fix host resolver being initialized after network requests arrive to the network service.

content/public/common/network_service_test.mojom

 // Copyright 2017 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.
 
 module content.mojom;
 
 struct Rule {
   string host_pattern;
   string replacement;
 };
 
 // Testing interface to the network service.
 interface NetworkServiceTest {
   // Adds the given host resolver rules in the process where the network
   // service is running.
-  AddRules(array<Rule> rules);
+  [Sync]
+  AddRules(array<Rule> rules) => ();
 };