[css-properties-values-api] Add test for cascading behavior.

Registered custom properties should behave as if they weren't registered
for parsing and cascading purposes. Only at computed-value time does the
syntax from the registration come into play.

R=chrishtr@chromium.org

Bug: 641877
Change-Id: I5ed5b334318e653da660b88b2dd912cc0b59f36d
Reviewed-on: https://chromium-review.googlesource.com/1136448
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575161}
1 file changed