[net] Add AsyncDns field trial.

net/base/host_resolver.h

Index: net/base/host_resolver.h
diff --git a/net/base/host_resolver.h b/net/base/host_resolver.h
index 58a50449d41f3d60df76870afc85d2ece713c971..d611b6dbd7ed6563573439a212f78225b70772d4 100644
--- a/net/base/host_resolver.h
+++ b/net/base/host_resolver.h
@@ -206,6 +206,14 @@ NET_EXPORT HostResolver* CreateNonCachingSystemHostResolver(
 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_