Allow 'alphabetic' for alignment-baseline and dominant-baseline

This keyword is not included in any of the ranges used, so needs to be
mentioned specifically.

BUG=620618

Review-Url: https://codereview.chromium.org/2131483003
Cr-Commit-Position: refs/heads/master@{#404338}
3 files changed