Add path_id to SerializedPacket. No functional 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 5e387449c43a985078df28923f7e08cdb1606987..e070ef63fa2b54b9664c1a33a24372efd38172b4 100644
--- a/net/quic/quic_connection.cc
+++ b/net/quic/quic_connection.cc
@@ -1729,6 +1729,7 @@ void QuicConnection::OnWriteError(int error_code) {
 }
 
 void QuicConnection::OnSerializedPacket(SerializedPacket* serialized_packet) {
+  DCHECK_NE(kInvalidPathId, serialized_packet->path_id);
   if (serialized_packet->packet == nullptr) {
     // We failed to serialize the packet, so close the connection.
     // CloseConnection does not send close packet, so no infinite loop here.