Move AsyncDns field trial to server-side control.

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.
+// Returns true if should set |HostResolver::Options::enable_async| to true.
+// Configures the AsyncDns field trial as follows:
+// groups AsyncDnsA and AsyncDnsB: return true
+// groups SystemDnsA and SystemDnsB: return false
+// otherwise (trial absent): return false
 bool ConfigureAsyncDnsFieldTrial();
 
 }  // namespace chrome_browser_net
 
 #endif  // CHROME_BROWSER_NET_ASYNC_DNS_FIELD_TRIAL_H_