StructuredDeserialize should be used; serialization records cannot be returned from IDL getters

[jeremyroman]

The current draft specifies that the values should be serialized, but never deserializes them.

The output of StructuredSerialize (and its kin, like StructuredSerializeForStorage) isn't a cloned ECMAScript value, but a realm-independent serialization record. It can't be returned as the result of a getter, but needs to be passed to StructuredDeserialize at some point to return a new (cloned) ECMAScript value in the appropriate realm. (If that's always the realm that made it, you might even invoke it immediately.)

[npm1]

@wanderview FYI I plan to call StructuredDeserialize immediately on the output of StructuredSerialize as I think that achieves the cloning we intended.