The ReviewRequestPageDataMixin was built on the BuiltinFieldMixin.

However, this requires that the field being represented be available on

the review request draft as field_id or else the mixin will use the

review request instead. This is not ideal when the represented field is

derived state or not directly accessible as an attribute/property on the

review request draft, e.g., it is an attribute of the draft diff

instead.
Now the two are more separate and fields that require the old behaviour

inherit from both.

Ran unit tests.