[webcrypto] Add PKCS#8 export for RSA private keys for NSS.

content/child/webcrypto/platform_crypto_openssl.cc

Index: content/child/webcrypto/platform_crypto_openssl.cc
diff --git a/content/child/webcrypto/platform_crypto_openssl.cc b/content/child/webcrypto/platform_crypto_openssl.cc
index 78054e313843f313d6c3a6c97033f585a7f677b5..ea89ea996846cf05b05f8eb63f71ef47446429a2 100644
--- a/content/child/webcrypto/platform_crypto_openssl.cc
+++ b/content/child/webcrypto/platform_crypto_openssl.cc
@@ -386,6 +386,13 @@ Status ExportKeySpki(PublicKey* key, blink::WebArrayBuffer* buffer) {
   return Status::ErrorUnsupported();
 }
 
+Status ExportKeyPkcs8(PrivateKey* key,
+                      const blink::WebCryptoKeyAlgorithm& key_algorithm,
+                      blink::WebArrayBuffer* buffer) {
+  // TODO(eroman): http://crbug.com/267888
+  return Status::ErrorUnsupported();
+}
+
 Status WrapSymKeyAesKw(SymKey* wrapping_key,
                        SymKey* key,
                        blink::WebArrayBuffer* buffer) {