Allow users of the |CRYPTO_BUFFER|-based methods to verify certs after the handshake.

Previously, the |CRYPTO_BUFFER|-based methods always rejected
certificate chains because none of the current callbacks is suitable to
use. In the medium-term, we want an async callback for this but, for
now, we would like to get Chromium working. Chromium already installs a
no-op callback (except for the logic that was moved into BoringSSL in
a58baaf9e68f887a9e1daf88cb25d911a376851f) and so this hack will suffice
for Chromium.

Change-Id: Ie44b7b32b9e42f503c47b072e958507754136d72
Reviewed-on: https://boringssl-review.googlesource.com/14125
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/ssl_lib.c

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 7ead554..856fba2 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -1598,6 +1598,10 @@
   ctx->signed_cert_timestamps_enabled = 1;
 }
 
+void SSL_CTX_i_promise_to_verify_certs_after_the_handshake(SSL_CTX *ctx) {
+  ctx->i_promise_to_verify_certs_after_the_handshake = 1;
+}
+
 void SSL_enable_signed_cert_timestamps(SSL *ssl) {
   ssl->signed_cert_timestamps_enabled = 1;
 }