[net] Add AsyncDns field trial.

chrome/browser/net/async_dns_field_trial.h

+// Copyright (c) 2012 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.
+
+#ifndef CHROME_BROWSER_NET_ASYNC_DNS_FIELD_TRIAL_H_
+#define CHROME_BROWSER_NET_ASYNC_DNS_FIELD_TRIAL_H_
+
+#include <string>
+
+class CommandLine;
+class Profile;
+
+namespace chrome_browser_net {
+
+// Configures the AsyncDns field trial as follows.
+// group A (async disabled): use CreateSystemHostResolver
+// group B (async enabled):  use CreateAsyncHostResolver
+// Returns true if should use CreateAsyncHostResolver.
+bool ConfigureAsyncDnsFieldTrial();
+
+}  // namespace chrome_browser_net
+
+#endif  // CHROME_BROWSER_NET_ASYNC_DNS_FIELD_TRIAL_H_