Implement the client side of certificate compression.

Change-Id: I0aced480af98276ebfe0970b4afb9aa957ee07cb
Reviewed-on: https://boringssl-review.googlesource.com/29024
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/tls1.h b/include/openssl/tls1.h
index bb9a816..03b218b 100644
--- a/include/openssl/tls1.h
+++ b/include/openssl/tls1.h
@@ -266,6 +266,10 @@
 #define TLSEXT_hash_sha384 5
 #define TLSEXT_hash_sha512 6
 
+// From https://tools.ietf.org/html/draft-ietf-tls-certificate-compression-03#section-3
+#define TLSEXT_cert_compression_zlib 1
+#define TLSEXT_cert_compression_brotli 2
+
 #define TLSEXT_MAXLEN_host_name 255
 
 // PSK ciphersuites from 4279