Log the quic version for internal server tracing. Not used in production.

net/quic/quic_connection.h

Index: net/quic/quic_connection.h
diff --git a/net/quic/quic_connection.h b/net/quic/quic_connection.h
index 74ed3bc3ca017a6316f46891b74c53ad8bfe801e..66681065acd6e4bab12cf13c486d3d061dbdf9e9 100644
--- a/net/quic/quic_connection.h
+++ b/net/quic/quic_connection.h
@@ -204,6 +204,9 @@ class NET_EXPORT_PRIVATE QuicConnectionDebugVisitor
 
   // Called when the connection is closed.
   virtual void OnConnectionClosed(QuicErrorCode error, bool from_peer) {}
+
+  // Called when the version negotiation is successful.
+  virtual void OnSuccessfulVersionNegotiation(const QuicVersion& version) {}
 };
 
 class NET_EXPORT_PRIVATE QuicConnectionHelperInterface {