Contradictory to the spec, the Set Timeouts endpoint should update the
existing timeouts configuration in-place, rather than replacing it. WPT
expects this, and other browsers already implement this endpoint this
way.
The spec's text is pretty awkward here, but the way we've currently
transcribed it to C++ means we reject valid script timeouts. This meant
the following would fail:
TimeoutsConfiguration config {}; // Default values.
auto json = timeouts_object(config);
config = TRY(json_deserialize_as_a_timeouts_configuration(json));
