Fix non-noise cpplint errors in //crypto.

crypto/hmac.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "crypto/hmac.h"
 
 #include <stddef.h>
 
 #include <algorithm>
+#include <string>
 
 #include "base/logging.h"
 #include "base/stl_util.h"
 #include "crypto/openssl_util.h"
 #include "crypto/secure_util.h"
 #include "crypto/symmetric_key.h"
 #include "third_party/boringssl/src/include/openssl/hmac.h"
 
 namespace crypto {
 
@@ skipping 64 matching lines @@
   std::unique_ptr<unsigned char[]> computed_digest(
       new unsigned char[digest_length]);
   if (!Sign(data, computed_digest.get(), digest_length))
     return false;
 
   return SecureMemEqual(digest.data(), computed_digest.get(),
                         std::min(digest.size(), digest_length));
 }
 
 }  // namespace crypto