Sync the browser test host resolver with the network process before a test runs.

content/public/common/network_service_test.mojom

Index: content/public/common/network_service_test.mojom
diff --git a/content/public/common/network_service_test.mojom b/content/public/common/network_service_test.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..451c15f42d429f325068f9ce16c24c6ed7a4afd1
--- /dev/null
+++ b/content/public/common/network_service_test.mojom
@@ -0,0 +1,17 @@
+// 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);
+};