Use SecKeyCreateSignature on macOS 10.12 and later.

net/ssl/ssl_platform_key.h

Index: net/ssl/ssl_platform_key.h
diff --git a/net/ssl/ssl_platform_key.h b/net/ssl/ssl_platform_key.h
index 00924001bce6eff7008188a316abd2624a86c3f5..efb42920ed655e2adf380d46df35df503147a5b2 100644
--- a/net/ssl/ssl_platform_key.h
+++ b/net/ssl/ssl_platform_key.h
@@ -15,11 +15,11 @@ namespace net {
 class SSLPrivateKey;
 class X509Certificate;
 
-// Looks up the private key from the platform key store corresponding to
-// |certificate|'s public key and returns an SSLPrivateKey backed by the
-// playform key.
+// Returns an SSLPrivateKey backed by the platform private key that corresponds
+// to |certificate|'s public key. If |keychain| is nullptr, the process's
+// default search list is used instead.
 NET_EXPORT scoped_refptr<SSLPrivateKey> FetchClientCertPrivateKey(
-    X509Certificate* certificate);
+    const X509Certificate* certificate);
 
 }  // namespace net