Refactor UnresolvedColorMix to support unresolved relative colors
Split off UnresolvedColorFunction as a base class of UnresolvedColorMix
and implement a new derived class UnresolvedRelativeColor to represent
unresolved relative colors.
Exercise the new class in a WPT by checking that we can serialize an
unresolved relative color out to computedStyleMap.
Bug: 325309578
Change-Id: Ie28ec93ac6b2f36735d619ef3a56290618b692ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5785061
Commit-Queue: Kevin Babbitt kbabbitt@microsoft.com
Reviewed-by: David Baron dbaron@chromium.org
Reviewed-by: Rune Lillesveen futhark@chromium.org
Cr-Commit-Position: refs/heads/main@{#1351129}