[css-properties-values-api] Validate var() fallbacks.

According to a recent spec edit, any fallback in a var()-reference must
match the syntax of the referenced property, otherwise the var()-reference
is invalid. This applies even if the fallback is not used.

To implement this, ResolveFallback now returns kNone/kFail/kSuccess instead
of a bool. This is necessary, because the kNone case may be both an error
state and a success state, depending on whether the fallback is being
used or not, hence a plain bool is not sufficient.

R=futhark@chromium.org

Bug: 641877
Change-Id: I951eef6335bf1cd4064d59959d73ec4fa39c9ee0
Reviewed-on: https://chromium-review.googlesource.com/c/1335576
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608365}
3 files changed