Add a QuicConfig option to ask the client not to attempt migration.

net/quic/crypto/crypto_protocol.h

Index: net/quic/crypto/crypto_protocol.h
diff --git a/net/quic/crypto/crypto_protocol.h b/net/quic/crypto/crypto_protocol.h
index 83df44d6b7eeeead4e29c6eac5f587c8e9362ef4..3a2cf3d1bc8ccf82e2b13a77ee75594f8e25de7b 100644
--- a/net/quic/crypto/crypto_protocol.h
+++ b/net/quic/crypto/crypto_protocol.h
@@ -102,6 +102,9 @@ const QuicTag kTCID = TAG('T', 'C', 'I', 'D');   // Connection ID truncation.
 // Multipath option.
 const QuicTag kMPTH = TAG('M', 'P', 'T', 'H');   // Enable multipath.
 
+const QuicTag kNCMR = TAG('N', 'C', 'M', 'R');   // Do not attempt connection
+                                                 // migration.
+
 // Enable bandwidth resumption experiment.
 const QuicTag kBWRE = TAG('B', 'W', 'R', 'E');  // Bandwidth resumption.
 const QuicTag kBWMX = TAG('B', 'W', 'M', 'X');  // Max bandwidth resumption.