[net/dns] Fix how DnsTransaction computes next server index after fallback.

net/dns/dns_transaction.cc

Index: net/dns/dns_transaction.cc
diff --git a/net/dns/dns_transaction.cc b/net/dns/dns_transaction.cc
index 6d4bf01217b19ecbcc572e79a86f556850b9c010..fdd2eb25dfca153bdc94848c587e89bbcd61e63c 100644
--- a/net/dns/dns_transaction.cc
+++ b/net/dns/dns_transaction.cc
@@ -651,8 +651,8 @@ class DnsTransactionImpl : public DnsTransaction,
 
     const DnsConfig& config = session_->config();
 
-    unsigned server_index = first_server_index_ +
-        (attempt_number % config.nameservers.size());
+    unsigned server_index =
+        (first_server_index_ + attempt_number) % config.nameservers.size();
 
     scoped_ptr<DnsSession::SocketLease> lease =
         session_->AllocateSocket(server_index, net_log_.source());