Web Animations CSS: Support animation of stroke-width

The existing CSS implementation allows negative stroke-width, but
Firefox does not, and nor does the spec. Change the behavior of
Web Animations CSS here to follow Firefox and the spec.

BUG=257591

Review URL: https://codereview.chromium.org/23691075

git-svn-id: svn://svn.chromium.org/blink/trunk@159183 bbb929c8-8fbe-4397-9dbb-9b2b20218538