CSS Animations: add interpolation test for border-radius

Border-radius should not be negative: interpolator fails for that case, and 
crashes in the virtual/web-animations-css/animations suite.

BUG=258896

Review URL: https://chromiumcodereview.appspot.com/23537013

git-svn-id: svn://svn.chromium.org/blink/trunk@157169 bbb929c8-8fbe-4397-9dbb-9b2b20218538