[css-ui] Update resolved value of caret-color property

The resolved value of caret-color should be the used value,
like for the rest of color properties.
This was discussed and resolved by the CSS WG
(see https://github.com/w3c/csswg-drafts/issues/781).

BUG=665422
TEST=imported/csswg-test/css-ui-3/caret-color-009.html
TEST=imported/csswg-test/css-ui-3/caret-color-013.html

Review-Url: https://codereview.chromium.org/2577633002
Cr-Commit-Position: refs/heads/master@{#442214}