[net] Add AsyncDns field trial.

net/base/host_resolver.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 NET_BASE_HOST_RESOLVER_H_
 #define NET_BASE_HOST_RESOLVER_H_
 
 #include <string>
 
 #include "net/base/address_family.h"
@@ skipping 188 matching lines @@
     size_t max_retry_attempts,
     NetLog* net_log);
 
 // As above, but the HostResolver will use the asynchronous DNS client in
 // DnsTransaction, which will be configured using DnsConfigService to match
 // the system DNS settings. If the client fails, the resolver falls back to
 // the global HostResolverProc.
 NET_EXPORT HostResolver* CreateAsyncHostResolver(size_t max_concurrent_resolves,
                                                  size_t max_retry_attempts,
                                                  NetLog* net_log);
+
+
+// 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.
+NET_EXPORT bool ConfigureAsyncDnsFieldTrial();
+
 }  // namespace net
 
 #endif  // NET_BASE_HOST_RESOLVER_H_