Web Animations: Fix timing model logic to correctly handle an iteration duration of infinity

Note that this path can't be triggered from CSS.

R=dstockwell

Review URL: https://codereview.chromium.org/85113002

git-svn-id: svn://svn.chromium.org/blink/trunk@162711 bbb929c8-8fbe-4397-9dbb-9b2b20218538