Add an API to record use of delegated credential

Change-Id: Ie964dee5ff9f8c6d43208dd1d3947d9b427ea27d
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/36424
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/ssl/tls13_both.cc b/ssl/tls13_both.cc
index bd0bb4f..2a290f4 100644
--- a/ssl/tls13_both.cc
+++ b/ssl/tls13_both.cc
@@ -497,6 +497,7 @@
       OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
       return 0;
     }
+    ssl->s3->delegated_credential_used = true;
   }
 
   for (size_t i = 1; i < sk_CRYPTO_BUFFER_num(cert->chain.get()); i++) {