Replace hash_current_message with get_current_message.

For TLS 1.3 draft 18, it will be useful to get at the full current
message and not just the body. Add a hook to expose it and replace
hash_current_message with a wrapper over it.

BUG=112

Change-Id: Ib9e00dd1b78e8b72e12409d85c80e96c5b411a8b
Reviewed-on: https://boringssl-review.googlesource.com/12238
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>
diff --git a/ssl/tls13_client.c b/ssl/tls13_client.c
index 4b0b56b..a5b2e85 100644
--- a/ssl/tls13_client.c
+++ b/ssl/tls13_client.c
@@ -379,7 +379,7 @@
   /* If there was no HelloRetryRequest, the version negotiation logic has
    * already hashed the message. */
   if (hs->received_hello_retry_request &&
-      !ssl->method->hash_current_message(ssl)) {
+      !ssl_hash_current_message(ssl)) {
     return ssl_hs_error;
   }
 
@@ -409,7 +409,7 @@
     return ssl_hs_error;
   }
 
-  if (!ssl->method->hash_current_message(ssl)) {
+  if (!ssl_hash_current_message(ssl)) {
     return ssl_hs_error;
   }
 
@@ -465,7 +465,7 @@
   sk_X509_NAME_pop_free(ssl->s3->hs->ca_names, X509_NAME_free);
   ssl->s3->hs->ca_names = ca_sk;
 
-  if (!ssl->method->hash_current_message(ssl)) {
+  if (!ssl_hash_current_message(ssl)) {
     return ssl_hs_error;
   }
 
@@ -477,7 +477,7 @@
                                                         SSL_HANDSHAKE *hs) {
   if (!tls13_check_message_type(ssl, SSL3_MT_CERTIFICATE) ||
       !tls13_process_certificate(ssl, 0 /* certificate required */) ||
-      !ssl->method->hash_current_message(ssl)) {
+      !ssl_hash_current_message(ssl)) {
     return ssl_hs_error;
   }
 
@@ -489,7 +489,7 @@
     SSL *ssl, SSL_HANDSHAKE *hs) {
   if (!tls13_check_message_type(ssl, SSL3_MT_CERTIFICATE_VERIFY) ||
       !tls13_process_certificate_verify(ssl) ||
-      !ssl->method->hash_current_message(ssl)) {
+      !ssl_hash_current_message(ssl)) {
     return 0;
   }
 
@@ -502,7 +502,7 @@
   static const uint8_t kZeroes[EVP_MAX_MD_SIZE] = {0};
   if (!tls13_check_message_type(ssl, SSL3_MT_FINISHED) ||
       !tls13_process_finished(ssl) ||
-      !ssl->method->hash_current_message(ssl) ||
+      !ssl_hash_current_message(ssl) ||
       /* Update the secret to the master secret and derive traffic keys. */
       !tls13_advance_key_schedule(ssl, kZeroes, hs->hash_len) ||
       !tls13_derive_traffic_secret_0(ssl)) {