Header validator inconsistent with spec re size checks

[apasel422]

Per https://wicg.github.io/attribution-reporting-api/#parse-attribution-destinations, the size of the destination field is supposed to be checked after URL-to-site transformation and deduplication, but the validator checks the size of the JSON list itself, causing some registrations that would be allowed by the spec to be prohibited by the validator.

This also applies to filter_data values.