Introduce QuicDatagramFlowId

This is currently only used by soon-to-be-added MASQUE code

gfe-relnote: n/a, add unused typedef
PiperOrigin-RevId: 279838432
Change-Id: Ic8d004dd729af1f954904cb0be63caf59db01d48

quic/core/quic_types.h

diff --git a/quic/core/quic_types.h b/quic/core/quic_types.h
index 06931dc..3511c8d 100644
--- a/quic/core/quic_types.h
+++ b/quic/core/quic_types.h
@@ -23,6 +23,7 @@
 typedef uint32_t QuicControlFrameId;
 typedef uint32_t QuicHeaderId;
 typedef uint32_t QuicMessageId;
+typedef uint64_t QuicDatagramFlowId;
 
 // TODO(fkastenholz): Should update this to 64 bits for V99.
 typedef uint32_t QuicStreamId;