Update some comments and code to reflect the fact that HMAC() is no longer hostile to NULL key data.

content/child/webcrypto/openssl/hmac_openssl.cc

Index: content/child/webcrypto/openssl/hmac_openssl.cc
diff --git a/content/child/webcrypto/openssl/hmac_openssl.cc b/content/child/webcrypto/openssl/hmac_openssl.cc
index 0872390d457a129fea36030889405ae4dbab9271..69b55bf0464979200a41f8bdb37be6aae8950e0a 100644
--- a/content/child/webcrypto/openssl/hmac_openssl.cc
+++ b/content/child/webcrypto/openssl/hmac_openssl.cc
@@ -38,22 +38,14 @@ Status SignHmac(const std::vector<uint8_t>& raw_key,
     return Status::ErrorUnsupported();
   unsigned int hmac_expected_length = EVP_MD_size(digest_algorithm);
 
-  // OpenSSL wierdness here.
-  // First, HMAC() needs a void* for the key data, so make one up front as a
-  // cosmetic to avoid a cast. Second, OpenSSL does not like a NULL key,
-  // which will result if the raw_key vector is empty; an entirely valid
-  // case. Handle this specific case by pointing to a fresh array.
-  const unsigned char null_key[] = {0};
-  const void* const raw_key_voidp = raw_key.size() ? &raw_key[0] : null_key;
-
   buffer->resize(hmac_expected_length);
   crypto::ScopedOpenSSLSafeSizeBuffer<EVP_MAX_MD_SIZE> hmac_result(
       vector_as_array(buffer), hmac_expected_length);
 
   unsigned int hmac_actual_length;
-  unsigned char* const success =
-      HMAC(digest_algorithm, raw_key_voidp, raw_key.size(), data.bytes(),
-           data.byte_length(), hmac_result.safe_buffer(), &hmac_actual_length);
+  unsigned char* const success = HMAC(
+      digest_algorithm, vector_as_array(&raw_key), raw_key.size(), data.bytes(),
+      data.byte_length(), hmac_result.safe_buffer(), &hmac_actual_length);
   if (!success || hmac_actual_length != hmac_expected_length)
     return Status::OperationError();