There are two things that should always be supported with any property:
- css-wide keywords like 'initial'
- var refs like 'var(--A)'

This patch adds tests to ensure that this works for all the different
types of properties. Unfortunately, supporting this in shorthands
is quite difficult (we get shorthands as strings, so to reify them
we have to parse, but we can't parse in reification) so we'll leave
that for the future.

Bug: 816722
Change-Id: I1d344b73d011db81cfc3880be54823302534fd36
Reviewed-on: https://chromium-review.googlesource.com/942121
Commit-Queue: Darren Shen <[email protected]>
Reviewed-by: nainar <[email protected]>
Cr-Commit-Position: refs/heads/master@{#540055}