This uses a `foo>bar` notation in the `valid-identifiers` field of
Properties.json, to say "replace `foo` with `bar`".
The motivation here is to avoid calling `parse_css_value_for_property()`
inside the per-property switch in `parse_css_value()`. Eventually we'll
need to be able to call that switch from
`parse_css_value_for_properties()` so that shorthands can make use of
any bespoke parsing code to parse their longhands.
943cc0e32a