Web Animations CSS: Fix clamping to non-negative values for BorderImageLength

Also beef up a number of tests that use this code

BUG=257591
R=alancutter@chromium.org, alancutter

Review URL: https://codereview.chromium.org/54123007

git-svn-id: svn://svn.chromium.org/blink/trunk@161971 bbb929c8-8fbe-4397-9dbb-9b2b20218538