Adding TLS 1.3 AEAD construction.

The TLS 1.3 spec has an explicit nonce construction for AEADs that
requires xoring the IV and sequence number.

Change-Id: I77145e12f7946ffb35ebeeb9b2947aa51058cbe9
Reviewed-on: https://boringssl-review.googlesource.com/8042
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/internal.h b/ssl/internal.h
index f3b6d01..5d003f9 100644
--- a/ssl/internal.h
+++ b/ssl/internal.h
@@ -283,6 +283,8 @@
   /* omit_version_in_ad is non-zero if the version should be omitted
    * in the AEAD's ad parameter. */
   char omit_version_in_ad;
+  /* omit_ad is non-zero if the AEAD's ad parameter should be omitted. */
+  char omit_ad;
   /* xor_fixed_nonce is non-zero if the fixed nonce should be XOR'd into the
    * variable nonce rather than prepended. */
   char xor_fixed_nonce;