Web Animations: Fix TimedItem to handle iterationDuration=0 and iterationCount=infinity

Currently this case hits an assertion in calculatePhase().

The spec is unclear about how this should be handled, but we need to do
something sensible as it's possible to hit this case from CSS with
'animation: myAnimation infinite'.

This change fixes TimedItem to handle this case such that the activeDuration is
zero and the animation immediately 'jumps' to the end of the iterations
(iterationCount=infinity, timeFraction=1).

Also removes some superfluous logic from calculateIterationTime() and
calculateCurrentIteration().

BUG=322575
R=dstockwell

Review URL: https://codereview.chromium.org/84953002

git-svn-id: svn://svn.chromium.org/blink/trunk@162668 bbb929c8-8fbe-4397-9dbb-9b2b20218538