Following #36977, the below note can be addressed:

by merging the two fields into one:

cross_realm_transform: Option<CrossRealmTransform>,

Where CrossRealmTransform would be defined as:

#[derive(Clone, JSTraceable, MallocSizeOf)]
enum CrossRealmTransform {
    Readable(CrossRealmTransformReadable),
    Writable(CrossRealmTransformWritable),
}

and where fixing the code using those concepts should be obvious.