Web Animations CSS: Support animation of -webkit-column-rule-width

The existing implementation allows negative widths. Here change CSS
Animations behavior to enforce the unsigned short range.

BUG=257591

Review URL: https://chromiumcodereview.appspot.com/23486011

git-svn-id: svn://svn.chromium.org/blink/trunk@158395 bbb929c8-8fbe-4397-9dbb-9b2b20218538