Add a merely pass-through QuicMultipathSentPacketManager. Protected behind blocked flag FLAGS_quic_…

net/quic/quic_utils.cc

Index: net/quic/quic_utils.cc
diff --git a/net/quic/quic_utils.cc b/net/quic/quic_utils.cc
index b3ba1a24df3151a4989d966536a8fd7e9d0c7493..9c0ca0c1c69a39205962e21ac8f3d110e8ac6760 100644
--- a/net/quic/quic_utils.cc
+++ b/net/quic/quic_utils.cc
@@ -299,6 +299,8 @@ const char* QuicUtils::ErrorToString(QuicErrorCode error) {
     RETURN_STRING_LITERAL(QUIC_ATTEMPT_TO_SEND_UNENCRYPTED_STREAM_DATA);
     RETURN_STRING_LITERAL(QUIC_MAYBE_CORRUPTED_MEMORY);
     RETURN_STRING_LITERAL(QUIC_CRYPTO_CHLO_TOO_LARGE);
+    RETURN_STRING_LITERAL(QUIC_MULTIPATH_PATH_DOES_NOT_EXIST);
+    RETURN_STRING_LITERAL(QUIC_MULTIPATH_PATH_NOT_ACTIVE);
     RETURN_STRING_LITERAL(QUIC_LAST_ERROR);
     // Intentionally have no default case, so we'll break the build
     // if we add errors and don't put them here.