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

content/child/webcrypto/platform_crypto.h

Index: content/child/webcrypto/platform_crypto.h
diff --git a/content/child/webcrypto/platform_crypto.h b/content/child/webcrypto/platform_crypto.h
index 3f884d60640e91cabac0ee781666af11f45cb319..8c28e3f02b9058427f474f89f38b8ae6a4f8e72e 100644
--- a/content/child/webcrypto/platform_crypto.h
+++ b/content/child/webcrypto/platform_crypto.h
@@ -177,6 +177,12 @@ Status ExportKeyRaw(SymKey* key, blink::WebArrayBuffer* buffer);
 Status ExportKeySpki(PublicKey* key, blink::WebArrayBuffer* buffer);
 
 // Preconditions:
+//  * |key| is non-null.
+Status ExportKeyPkcs8(PrivateKey* key,
+                      const blink::WebCryptoKeyAlgorithm& key_algorithm,
+                      blink::WebArrayBuffer* buffer);
+
+// Preconditions:
 //  * |wrapping_key| is non-null
 //  * |key| is non-null
 Status WrapSymKeyAesKw(SymKey* wrapping_key,