Make var() and attr() short-circuiting

Following the resolution in Issue 11500, all substitution functions
should be short-circuiting. This CL implements this for var()
and attr(), behind the flag CSSShortCircuitVarAttr.

Note that env() already has the desired behavior, and therefore
no change is needed for env().

https://github.com/w3c/csswg-drafts/issues/11500

Bug: 397690639
Change-Id: If71326bc52a55524ecfd4d439d30ac09bf22fea3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6530122
Reviewed-by: Munira Tursunova <moonira@google.com>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460882}