CSS Animations: Fix rounding in interpolation test helper

Previously values such as 0.999 were being 'rounded' to 0.1

Review URL: https://chromiumcodereview.appspot.com/23547015

git-svn-id: svn://svn.chromium.org/blink/trunk@157876 bbb929c8-8fbe-4397-9dbb-9b2b20218538