Web Animations: Replace hasIterationDuration with a NaN duration value

Minor clean-up to the code; e.g. this makes writing tests a bit simpler.

BUG=335350

Review URL: https://codereview.chromium.org/154333003

git-svn-id: svn://svn.chromium.org/blink/trunk@166567 bbb929c8-8fbe-4397-9dbb-9b2b20218538