[w3c/payment-request] retry(errorFields) should be optional (#804)

Small bug that only affects Gecko, I think. 

WebIDL says:

> If the type of an argument is a dictionary type or a union type that has a dictionary as one of its flattened member types, and that dictionary type and its ancestors have no required members, and the argument is either the final argument or is followed only by optional arguments, then the argument must be specified as optional.

Thus, `retry()` should be `retry(optional PaymentValidationErrors errorFields)`. 

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