Move AsyncDns field trial to server-side control.

chrome/browser/net/async_dns_field_trial.h

Index: chrome/browser/net/async_dns_field_trial.h
diff --git a/chrome/browser/net/async_dns_field_trial.h b/chrome/browser/net/async_dns_field_trial.h
index 48b3945fb0bbebb5ba12d46d17553abcbaf813cb..8d72e181c0e7946ce66c7df8467e641456715714 100644
--- a/chrome/browser/net/async_dns_field_trial.h
+++ b/chrome/browser/net/async_dns_field_trial.h
@@ -12,10 +12,11 @@ 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