Group sending path response and reverse path challenge closer on the server side.

The advantages are:
(1) The outermost ScopedPeerAddressContext can be instantiated after alternative_path_ has been created. This makes it straightforward to find a connection ID for this scoped context.
(2) Ideally, path response and reverse path challenge is better sent at the end of packet processing. But that is a more complex change. The current change allows progress made in usage of connection ID without that.

Protected by FLAGS_quic_reloadable_flag_quic_group_path_response_and_challenge_sending_closer.

PiperOrigin-RevId: 368275656
Change-Id: I1735ff20babd2f190ba27ea7037e2be9317a6f3a
diff --git a/quic/core/quic_connection.h b/quic/core/quic_connection.h
index 5513795..b935943 100644
--- a/quic/core/quic_connection.h
+++ b/quic/core/quic_connection.h
@@ -2229,6 +2229,14 @@
   bool use_connection_id_on_default_path_ =
       GetQuicReloadableFlag(quic_use_connection_id_on_default_path);
 
+  // Indicates whether we should proactively validate peer address on a
+  // PATH_CHALLENGE received.
+  bool should_proactively_validate_peer_address_on_path_challenge_ = false;
+
+  const bool group_path_response_and_challenge_sending_closer_ =
+      GetQuicReloadableFlag(
+          quic_group_path_response_and_challenge_sending_closer);
+
   const bool quic_deprecate_incoming_connection_ids_ =
       GetQuicReloadableFlag(quic_deprecate_incoming_connection_ids);
 };