Fix build error in chromium by adding QUIC_EXPORT.

gfe-relnote: n/a (not used in prod)
PiperOrigin-RevId: 276163355
Change-Id: If8ad521fff7ebf38af196d08ed9f52d5a6e43729

quic/quic_transport/quic_transport_stream.h

diff --git a/quic/quic_transport/quic_transport_stream.h b/quic/quic_transport/quic_transport_stream.h
index 11ea2dc..3b94eab 100644
--- a/quic/quic_transport/quic_transport_stream.h
+++ b/quic/quic_transport/quic_transport_stream.h
@@ -19,7 +19,7 @@
 // QuicTransportStream is an extension of QuicStream that provides I/O interface
 // that is safe to use in the QuicTransport context.  The interface ensures no
 // application data is processed before the client indication is processed.
-class QuicTransportStream : public QuicStream {
+class QUIC_EXPORT QuicTransportStream : public QuicStream {
  public:
   class Visitor {
    public: