This will be inherited by "legacy platform objects", i.e objects that
need to hijack indexed and/or named property access as described in the
IDL spec: https://webidl.spec.whatwg.org/#dfn-legacy-platform-object
Instead of overriding JS::Object virtuals, subclasses only need to
implement a very simple interface for property queries.
Note that this code is taken verbatim from code generator output.
I didn't write any of this now, so it's effectively "moved" code.
