[css-properties-values-api] Substitute initial values as computed values.

Registered custom properties substitute as token sequences equivalent to
their computed values. We already do this correctly for non-initial values,
but when substituting the initial value of a registered custom property,
we would substitute the original token sequence instead of the computed
value equivalent.

R=futhark@chromium.org

Bug: 641877
Change-Id: Icc9b8844d98cefd030be0daa91aef7963d55d7bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1517700
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640268}
5 files changed