Consolidate logic for resolving color-mix

Delegate resolution at computed-value time to the same path we call at
used-value time. This ensures consistent resolution behavior between the
two.

Also clarify in comments that sub-expressions in nested color functions
should be resolved at computed-value time per spec and add unit tests
to validate. (This won't be testable in WPTs until crbug.com/40137692
is fixed.)

Bug: 365818844
Change-Id: I7b199375c730b6aeb779829a9b5ae670e7c724c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5850660
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Kevin Babbitt <kbabbitt@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1355997}