[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 5fdee3965c814dd3959ab692829af26292d48b0c..30825a89b219bee5ed0a51ee4d8e456af4096fa5 100644
--- a/content/child/webcrypto/platform_crypto_openssl.cc
+++ b/content/child/webcrypto/platform_crypto_openssl.cc
@@ -387,6 +387,13 @@ 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) {