CSS Animations: add interpolation test for min-width

CSS min-width should not be negative: interpolator fails for that case.

Review URL: https://chromiumcodereview.appspot.com/23795003

git-svn-id: svn://svn.chromium.org/blink/trunk@157346 bbb929c8-8fbe-4397-9dbb-9b2b20218538