Fix crypto_unittests on Windows BoringSSL port.

MSVC is somewhat pickier about casting things to bool.

BUG=338884

Review URL: https://codereview.chromium.org/618123007

Cr-Commit-Position: refs/heads/master@{#297770}
diff --git a/crypto/signature_creator_openssl.cc b/crypto/signature_creator_openssl.cc
index 51cb3c3e..7a1349b5 100644
--- a/crypto/signature_creator_openssl.cc
+++ b/crypto/signature_creator_openssl.cc
@@ -68,9 +68,8 @@
   signature->resize(RSA_size(rsa_key.get()));
 
   unsigned int len = 0;
-  bool success = RSA_sign(ToOpenSSLDigestType(hash_alg), data, data_len,
-                          vector_as_array(signature), &len, rsa_key.get());
-  if (!success) {
+  if (!RSA_sign(ToOpenSSLDigestType(hash_alg), data, data_len,
+                vector_as_array(signature), &len, rsa_key.get())) {
     signature->clear();
     return false;
   }