Wire in the ack exponent trransport config for IETF QUIC

gfe-relnote: N/A, for IETF QUIC Only, protected by V99 flag.
PiperOrigin-RevId: 260736433
Change-Id: I5468bc4f052d78d2bf6890cdf0734cece5b107c0
diff --git a/quic/core/quic_constants.h b/quic/core/quic_constants.h
index 1ba32d8..cdee0f9 100644
--- a/quic/core/quic_constants.h
+++ b/quic/core/quic_constants.h
@@ -113,6 +113,9 @@
 // in low-bandwidth (< ~384 kbps), where an ack is sent per packet.
 const int64_t kDefaultDelayedAckTimeMs = 25;
 
+// Default shift of the ACK delay in the IETF QUIC ACK frame.
+const uint32_t kDefaultAckDelayExponent = 3;
+
 // Minimum tail loss probe time in ms.
 static const int64_t kMinTailLossProbeTimeoutMs = 10;