Though this may make us use more space when serializing
StyleTransform, but we don't have to do extra conversion on the compostior
side, and this makes us easier to maintain the Rust type.
Differential Revision: https://phabricator.services.mozilla.com/D60045
This is just a refactor in the right direction. Eventual goal is:
* All inherited properties use ArcSlice<>.
* All reset properties use OwnedSlice<> (or ThinVec<>).
No conversion happens at all, so we can remove all that glue, and also
compute_iter and co.
Of course there's work to do, but this is a step towards that.
Differential Revision: https://phabricator.services.mozilla.com/D30127