record inside union is not handled correctly in overload resolution algorithm

[bzbarsky]

Consider this idl:

void foo((Node or record<DOMString, Node>) arg);
void foo(DOMString arg);

and a call like so:

foo(null);

Which overload would be selected? If a dictionary were used instead of a record, the first overload would be selected. But with the current spec text for record, the second overload will be selected, as far as I can tell: https://heycam.github.io/webidl/#es-overloads step 11 substep 3 the last item doesn't include records, only dictionaries.

@jyasskin, @domenic, @tobie

[bzbarsky]

This is fallout from #180