Tidy DTLS cookie callback types.

const-correctness, unsigned long -> size_t.

Change-Id: Ic0c2685a48a0f98396c5753b6077c6c0c3b92326
Reviewed-on: https://boringssl-review.googlesource.com/1540
Reviewed-by: Adam Langley <agl@google.com>

ssl/s3_srvr.c

diff --git a/ssl/s3_srvr.c b/ssl/s3_srvr.c
index 76766b0..6a028ef 100644
--- a/ssl/s3_srvr.c
+++ b/ssl/s3_srvr.c
@@ -1010,8 +1010,7 @@
 			if (s->ctx->app_verify_cookie_cb != NULL)
 				{
 				if (s->ctx->app_verify_cookie_cb(s,
-						(unsigned char*)CBS_data(&cookie),
-						CBS_len(&cookie)) == 0)
+						CBS_data(&cookie), CBS_len(&cookie)) == 0)
 					{
 					al=SSL_AD_HANDSHAKE_FAILURE;
 					OPENSSL_PUT_ERROR(SSL, ssl3_get_client_hello, SSL_R_COOKIE_MISMATCH);