Web Animations: Fix calculation of transformed time when iteration duration is infinity

This was recently fixed in the spec in https://dvcs.w3.org/hg/FXTF/rev/449620dc5b67

R=dstockwell

Review URL: https://codereview.chromium.org/116093002

git-svn-id: svn://svn.chromium.org/blink/trunk@163942 bbb929c8-8fbe-4397-9dbb-9b2b20218538