[webcrypto] Add JWK import/export of RSA private keys (NSS).

content/child/webcrypto/shared_crypto.h

Index: content/child/webcrypto/shared_crypto.h
diff --git a/content/child/webcrypto/shared_crypto.h b/content/child/webcrypto/shared_crypto.h
index 47c623b64db1be4ddde8c0374cc8e149a90f8e8e..57627d60b0dad359a47ce5a620517ab5d95a9fd2 100644
--- a/content/child/webcrypto/shared_crypto.h
+++ b/content/child/webcrypto/shared_crypto.h
@@ -160,6 +160,20 @@ CONTENT_EXPORT bool DeserializeKeyForClone(
     const CryptoData& key_data,
     blink::WebCryptoKey* key);
 
+namespace platform {
+class SymKey;
+class PublicKey;
+class PrivateKey;
+}
+
+Status ToPlatformSymKey(const blink::WebCryptoKey& key, platform::SymKey** out);
+
+Status ToPlatformPublicKey(const blink::WebCryptoKey& key,
+                           platform::PublicKey** out);
+
+Status ToPlatformPrivateKey(const blink::WebCryptoKey& key,
+                            platform::PrivateKey** out);
+
 }  // namespace webcrypto
 
 }  // namespace content