Web Animations: End exclusive behavior at time 0 when playing backwards

Whether at animation is in effect at current time 0 while playing backwards
should be end-exclusive and depend on the specified fill mode.

Review URL: https://codereview.chromium.org/597463002

git-svn-id: svn://svn.chromium.org/blink/trunk@182473 bbb929c8-8fbe-4397-9dbb-9b2b20218538