Downgrading rst stream codes which wouldn't be recognized by the peer.

net/quic/quic_protocol.h

Index: net/quic/quic_protocol.h
diff --git a/net/quic/quic_protocol.h b/net/quic/quic_protocol.h
index c614d7e2cb4308a1b3ae425f8a151d3fd40658e0..852ecedc090940d68956692cd0d7b04150984dd7 100644
--- a/net/quic/quic_protocol.h
+++ b/net/quic/quic_protocol.h
@@ -356,6 +356,13 @@ enum QuicRstStreamErrorCode {
   QUIC_STREAM_LAST_ERROR,
 };
 
+// Because receiving an unknown QuicRstStreamErrorCode results in connection
+// teardown, we use this to make sure any errors predating a given version are
+// downgraded to the most appropriate existing error.
+QuicRstStreamErrorCode AdjustErrorForVersion(
+    QuicRstStreamErrorCode error_code,
+    QuicVersion version);
+
 // These values must remain stable as they are uploaded to UMA histograms.
 // To add a new error code, use the current value of QUIC_LAST_ERROR and
 // increment QUIC_LAST_ERROR.