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

net/quic/quic_sent_packet_manager.h

Index: net/quic/quic_sent_packet_manager.h
diff --git a/net/quic/quic_sent_packet_manager.h b/net/quic/quic_sent_packet_manager.h
index f500cb860a026c5d79726774b1057ae1d854111c..61695d44a156647622f8e0a1435b603fefc387f0 100644
--- a/net/quic/quic_sent_packet_manager.h
+++ b/net/quic/quic_sent_packet_manager.h
@@ -102,6 +102,12 @@ class NET_EXPORT_PRIVATE QuicSentPacketManager {
     // Called when RTT may have changed, including when an RTT is read from
     // the config.
     virtual void OnRttChange() = 0;
+
+    // Called with the path may be degrading. Note that the path may only be
+    // temporarily degrading.
+    // TODO(jri): With multipath, this method should probably have a path_id
+    // parameter, and should maybe result in the path being marked as inactive.
+    virtual void OnPathDegrading() = 0;
   };
 
   QuicSentPacketManager(Perspective perspective,