Implement TLS client auth in the OS X OpenSSL port.

net/ssl/openssl_platform_key.h

+// Copyright 2014 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.
+
+#ifndef NET_SSL_OPENSSL_PLATFORM_KEY_H_
+#define NET_SSL_OPENSSL_PLATFORM_KEY_H_
+
+#include "crypto/scoped_openssl_types.h"
+
+namespace net {
+
+class X509Certificate;
+
+// Looks up the private key from the platform key store corresponding
+// to |certificate|'s public key. Then wraps it in an OpenSSL EVP_PKEY
+// structure to be used for SSL client auth.
+//
+// TODO(davidben): This combines looking up a private key with
+// wrapping it in an OpenSSL structure. This will be separated with
+// https://crbug.com/394131
+crypto::ScopedEVP_PKEY FetchClientCertPrivateKey(
+    const X509Certificate* certificate);
+
+}  // namespace net
+
+#endif  // NET_SSL_OPENSSL_PLATFORM_KEY_H_