Web Animations: Fix CSS events to handle animations with very short durations

In order to do this reliably we need to handle the case where an animation
starts and ends within a single sample.

This is tested with a new LayoutTest which uses a CSS animation with a very 
short duration and a non-zero delay, as animations are always sampled at an
inherited time of zero.

BUG=240653
R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/23173007

git-svn-id: svn://svn.chromium.org/blink/trunk@156546 bbb929c8-8fbe-4397-9dbb-9b2b20218538