The alias is left there temporarilly and will be removed completely in a later commit where
also components/style/gecko/generated/structs_{debug|release}.rs are re-generated (they still
use the old alias).
We need another flag that represents allow-negative-number for SMIL, so
this enum will also comprise the another parsing mode that allows negative number.
To make it possible to check the rule type when parsing lengths, we need to pass
the `ParserContext` down through many layers to the place where length units are
parsed.
This change leaves it unused, so it's only to prepare for the next change.
MozReview-Commit-ID: 70YwtcCxnWw
