Support building BoringSSL with NSS certificates.

net/ssl/openssl_platform_key_nss.cc

Index: net/ssl/openssl_platform_key_nss.cc
diff --git a/net/ssl/openssl_platform_key_nss.cc b/net/ssl/openssl_platform_key_nss.cc
new file mode 100644
index 0000000000000000000000000000000000000000..8f3d554a857f72eac3887c6f6d20fe3e41ef91dd
--- /dev/null
+++ b/net/ssl/openssl_platform_key_nss.cc
@@ -0,0 +1,17 @@
+// Copyright 2015 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 "base/logging.h"
+#include "net/ssl/openssl_platform_key.h"
+
+namespace net {
+
+crypto::ScopedEVP_PKEY FetchClientCertPrivateKey(
+    const X509Certificate* certificate) {
+  // TODO(davidben): Implement client auth for NSS.
+  NOTIMPLEMENTED();
+  return nullptr;
+}
+
+}  // namespace net