Use SecKeyCreateSignature on macOS 10.12 and later.

net/ssl/ssl_platform_key_mac.h

+// Copyright 2016 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_SSL_PLATFORM_KEY_MAC_H_
+#define NET_SSL_SSL_PLATFORM_KEY_MAC_H_
+
+#include <Security/SecBase.h>
+
+#include "base/memory/ref_counted.h"
+#include "net/base/net_export.h"
+
+namespace net {
+
+class SSLPrivateKey;
+class X509Certificate;
+
+// Returns an SSLPrivateKey backed by the platform private key in |keychain|
+// that corresponds to |certificate|'s public key. If |keychain| is nullptr, the
+// user's default search list is used instead.
+NET_EXPORT_PRIVATE scoped_refptr<SSLPrivateKey>
+FetchClientCertPrivateKeyFromKeychain(const X509Certificate* certificate,
+                                      SecKeychainRef keychain);
+
+}  // namespace net
+
+#endif  // NET_SSL_SSL_PLATFORM_KEY_MAC_H_