ToJSVal for webidl dictionaries ignores the parent field

[jdm]

dictionary RTCRtpParameters {
  required sequence<RTCRtpHeaderExtensionParameters> headerExtensions;
  required RTCRtcpParameters rtcp;
  required sequence<RTCRtpCodecParameters> codecs;
};

dictionary RTCRtpSendParameters : RTCRtpParameters {
  required DOMString transactionId;
  required sequence<RTCRtpEncodingParameters> encodings;
};

When there is code that calls RTCRtpSender.getParameters() which returns RTCRtpSendParameters, we end up with a JS value that only includes transactionId and encodings, and none of the fields from RTCRtpParameters.