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>

ssl/ssl_cert.cc

diff --git a/ssl/ssl_cert.cc b/ssl/ssl_cert.cc
index 54df38f..b565a35 100644
--- a/ssl/ssl_cert.cc
+++ b/ssl/ssl_cert.cc
@@ -1010,3 +1010,7 @@
 
   return cert_set_dc(ssl->config->cert.get(), dc, pkey, key_method);
 }
+
+int SSL_delegated_credential_used(const SSL *ssl) {
+  return ssl->s3->delegated_credential_used;
+}