CSS Animations: Use more accurate timing functions in interpolation tests

Specifically this ensures that we test at exactly 0 and 1 when specified.

Review URL: https://codereview.chromium.org/25909003

git-svn-id: svn://svn.chromium.org/blink/trunk@159192 bbb929c8-8fbe-4397-9dbb-9b2b20218538