This affects connection migration only: 

Currently QuicChromiumClientSession is closed after migration error if the migration cause is path degrading.

MigrationResult QuicStreamFactory::MaybeMigrateSingleSessionOnPathDegrading() {
  ...
  if (migrate_sessions_early_) {
    result = MaybeMigrateSingleSession(session, /*close_session_on_error*/true, migration_net_log);
  }
  ...
}

However, OnPathDegrading is QUIC layer speculative network quality diagnosis, it needs to be tuned and might introduce false positive. Instead of close the session after migration error, it might be better to abort migration and leave the session open. We will close session if 5RTO fires.