Fix IDL definition of `LoadDocumentCallback`

[tidoust]

Dictionary arguments cannot be nullable in Web IDL:

"Although dictionary types can in general be nullable, they cannot when used as the type of an operation argument or a dictionary member." https://webidl.spec.whatwg.org/#dfn-nullable-type

Note: this was missed before because the webidl2 checker did not validate arguments in callback definitions. It now does.

---

Preview | Diff

[gkellogg]

Thanks, although this may be a controversial opinion, I think it is low-enough impact to be considered a class-2 change.