Web Animations CSS: max- and min- widths and heights can't be negative

CSS max- and min- widths and heights can't be negative [1]. Make
Web Animations CSS follow the spec: use animatableValueToLength()
with option NonNegativeValues to set these properties.

[1] http://www.w3.org/TR/2011/REC-CSS2-20110607/visudet.html#min-max-widths

BUG=257591

Review URL: https://codereview.chromium.org/26129012

git-svn-id: svn://svn.chromium.org/blink/trunk@159427 bbb929c8-8fbe-4397-9dbb-9b2b20218538