Enable sending ACK-ECN frames for IETF interop.

PiperOrigin-RevId: 502968927

quiche/quic/core/quic_versions.cc

diff --git a/quiche/quic/core/quic_versions.cc b/quiche/quic/core/quic_versions.cc
index 70c5549..1f03b70 100644
--- a/quiche/quic/core/quic_versions.cc
+++ b/quiche/quic/core/quic_versions.cc
@@ -644,6 +644,7 @@
 
 void QuicVersionInitializeSupportForIetfDraft() {
   // Enable necessary flags.
+  SetQuicRestartFlag(quic_receive_ecn, true);
 }
 
 void QuicEnableVersion(const ParsedQuicVersion& version) {