gfe-relnote: Let GFE be able to use HTTP2(tree-style) priority write scheduler in QUIC and enable it via a connection option H2PR. Protected by gfe2_reloadable_flag_quic_use_http2_priority_write_scheduler.

PiperOrigin-RevId: 260938733
Change-Id: I6d3f6c325a07b17bdfd8c416add8327b8d54be8a
diff --git a/quic/core/crypto/crypto_protocol.h b/quic/core/crypto/crypto_protocol.h
index 12afbd1..81f9730 100644
--- a/quic/core/crypto/crypto_protocol.h
+++ b/quic/core/crypto/crypto_protocol.h
@@ -197,6 +197,9 @@
 const QuicTag kMTUH = TAG('M', 'T', 'U', 'H');  // High-target MTU discovery.
 const QuicTag kMTUL = TAG('M', 'T', 'U', 'L');  // Low-target MTU discovery.
 
+// Enable Priority scheme experiment.
+const QuicTag kH2PR = TAG('H', '2', 'P', 'R');  // HTTP2 priorities.
+
 // Proof types (i.e. certificate types)
 // NOTE: although it would be silly to do so, specifying both kX509 and kX59R
 // is allowed and is equivalent to specifying only kX509.