Adds plumbing for QUIC early connection migration. No behavior change.

net/quic/quic_connection.cc

Index: net/quic/quic_connection.cc
diff --git a/net/quic/quic_connection.cc b/net/quic/quic_connection.cc
index 6c6a1f27757805f2ee97a6f632e77c07c2fb4881..db15e0aac99dd81cf86c203b041902f544673c50 100644
--- a/net/quic/quic_connection.cc
+++ b/net/quic/quic_connection.cc
@@ -1803,6 +1803,10 @@ void QuicConnection::OnRttChange() {
   packet_generator_.OnRttChange(rtt);
 }
 
+void QuicConnection::OnPathDegrading() {
+  visitor_->OnPathDegrading();
+}
+
 void QuicConnection::OnHandshakeComplete() {
   sent_packet_manager_.SetHandshakeConfirmed();
   // The client should immediately ack the SHLO to confirm the handshake is