Preserve the peer signature algorithm across resumes.

So we can report it cleanly out of DevTools, it should behave like
SSL_get_curve_id and be reported on resumption too.

BUG=chromium:658905

Change-Id: I0402e540a1e722e09eaebadf7fb4785d8880c389
Reviewed-on: https://boringssl-review.googlesource.com/12694
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index 6c1ec5b..cffa827 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -3669,6 +3669,10 @@
    * if not applicable or unknown. */
   uint16_t group_id;
 
+  /* peer_signature_algorithm is the signature algorithm used to authenticate
+   * the peer, or zero if not applicable or unknown. */
+  uint16_t peer_signature_algorithm;
+
   /* master_key, in TLS 1.2 and below, is the master secret associated with the
    * session. In TLS 1.3 and up, it is the resumption secret. */
   int master_key_length;