Implement working tests for the simple QUIC client classes.

net/quic/quic_data_stream.h

Index: net/quic/quic_data_stream.h
diff --git a/net/quic/quic_data_stream.h b/net/quic/quic_data_stream.h
index c80cacf662b9d93fe04bc7234961d7c31f2c306c..7bef3375f88a04289f041a16869551d4bf37f6c7 100644
--- a/net/quic/quic_data_stream.h
+++ b/net/quic/quic_data_stream.h
@@ -39,7 +39,7 @@ class SSLInfo;
 class NET_EXPORT_PRIVATE QuicDataStream : public ReliableQuicStream {
  public:
   // Visitor receives callbacks from the stream.
-  class Visitor {
+  class NET_EXPORT_PRIVATE Visitor {
    public:
     Visitor() {}