ServoStyleSetSizes now has two uses, one for the Stylist, and one for the UA
cache, and so the patch removes 'Stylist' from the field names.
Example output from about:memory:
> +----1,359,608 B (00.55%) -- layout
> | +----756,488 B (00.31%) -- style-sheet-cache [2]
> | +----393,968 B (00.16%) -- servo-ua-cache
> | | +--234,496 B (00.10%) -- element-and-pseudos-maps
> | | +---59,648 B (00.02%) -- revalidation-selectors
> | | +---58,320 B (00.02%) -- invalidation-map
> | | +---30,752 B (00.01%) -- other
> | | +---10,752 B (00.00%) -- precomputed-pseudos
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).