Add experimental TLS 1.3 short record header extension.

This extension will be used to test whether
https://github.com/tlswg/tls13-spec/pull/762 is deployable against
middleboxes. For simplicity, it is mutually exclusive with 0-RTT. If
client and server agree on the extension, TLS 1.3 records will use the
format in the PR rather than what is in draft 18.

BUG=119

Change-Id: I1372ddf7b328ddf73d496df54ac03a95ede961e1
Reviewed-on: https://boringssl-review.googlesource.com/12684
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>

ssl/internal.h

diff --git a/ssl/internal.h b/ssl/internal.h
index a4ae83a..7a10936 100644
--- a/ssl/internal.h
+++ b/ssl/internal.h
@@ -1483,6 +1483,10 @@
    * handshake. */
   unsigned tlsext_channel_id_valid:1;
 
+  /* short_header is one if https://github.com/tlswg/tls13-spec/pull/762 has
+   * been negotiated. */
+  unsigned short_header:1;
+
   uint8_t send_alert[2];
 
   /* pending_message is the current outgoing handshake message. */