Implement ContentType TLS 1.3 variant.

This implements PR #1051
(https://github.com/tlswg/tls13-spec/pull/1051).

Local experiments were not able to replicate the claims in the PR, but
implement this anyway for comparison purposes.

Change-Id: Ic9baf5e671f9a44565020466a553dd08f5ec0f1b
Reviewed-on: https://boringssl-review.googlesource.com/17844
Reviewed-by: Steven Valdez <svaldez@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/include/openssl/ssl3.h b/include/openssl/ssl3.h
index 98648c4..2b241ba 100644
--- a/include/openssl/ssl3.h
+++ b/include/openssl/ssl3.h
@@ -272,6 +272,7 @@
 #define SSL3_RT_ALERT 21
 #define SSL3_RT_HANDSHAKE 22
 #define SSL3_RT_APPLICATION_DATA 23
+#define SSL3_RT_PLAINTEXT_HANDSHAKE 24
 
 /* Pseudo content type for SSL/TLS header info */
 #define SSL3_RT_HEADER 0x100