Otherwise, `margin: var(--foo) var(--bar)` would be wrongly serialized
as `margin: var(--foo)var(--bar)`
(cherry picked from commit 6e68e8f3c9d74893e46ab0514169b950866457b4)
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)
