[css-backgrounds] Always serialize 'background-size' and `mask-size` as two values

See https://github.com/whatwg/compat/issues/28

That change is now implemented in WebKit: https://github.com/WebKit/WebKit/pull/3372

Relevant CSS specs: https://drafts.csswg.org/css-backgrounds-3/#background-size and https://drafts.csswg.org/cssom/#serialize-a-css-value

@pxlcoder asked

Do we have consensus from other engines on:

Always serialize background-size as two values.

CSSOM says

• If component values can be omitted or replaced with a shorter representation without changing the meaning of the value, omit/replace them.

• If either of the above syntactic translations would be less backwards-compatible, do not perform them.

The last bullet point seems to apply here, as it impacts -webkit-background-size if implemented as a legacy shorthand. But maybe we can include each known property with special serialization, and what that serialization should be, so it's not ambiguous for those properties.

@pxlcoder also asked:

Should we also change the serialization of mask-size to always use two values, for consistency?

Since mask-size also has a -webkit- prefixed variant and also uses <bg-size># value grammar, I think it makes sense to use the same serialization rule as for background-size.

Start with the CSS Backgrounds and CSSOM serialization sections linked in the issue, then review the WHATWG compatibility discussion and WebKit change. Determine the agreed serialization rules for background-size and mask-size, including their prefixed variants, and document the consensus in the relevant specification text.