A couple of parts of this:
- Store the source text for Declarations of custom properties.
- Then save that in the UnresolvedStyleValue.
- Serialize UnresolvedStyleValue using the saved source when available -
that is, for custom properties but not for regular properties that
include var() or attr().
(cherry picked from commit bf3e6daedbc8fd0d8e03f5f0eafebf5bbe3abf03)