gfe-relnote: Don't DCHECK in GetPacketHeaderSize if the flag is false. Protected by reloadable flag quic_fix_get_packet_header_size

PiperOrigin-RevId: 256072474
Change-Id: I4066ec1bd90ab29188448103857a9eaa526c235c

quic/core/quic_packets.cc

diff --git a/quic/core/quic_packets.cc b/quic/core/quic_packets.cc
index c6cef78..4887e82 100644
--- a/quic/core/quic_packets.cc
+++ b/quic/core/quic_packets.cc
@@ -135,6 +135,7 @@
         size += kDiversificationNonceSize;
       }
       DCHECK(QuicVersionHasLongHeaderLengths(version) ||
+             !GetQuicReloadableFlag(quic_fix_get_packet_header_size) ||
              retry_token_length_length + retry_token_length + length_length ==
                  0);
       if (QuicVersionHasLongHeaderLengths(version) ||