Clarify the current iteration index = infinity becomes infinity when exposed to ECMAScript

As per discussion:
http://lists.w3.org/Archives/Public/public-fx/2014AprJun/0141.html

Generated from:

commit f2516a8a4062a88de956c34c5bfd2bc5789f200e
Author: Brian Birtles <birtles@gmail.com>
Date:   Thu Sep 25 16:33:50 2014 +0900

    Clarify the current iteration index = infinity becomes infinity when exposed to ECMAScript

    As per discussion:
    http://lists.w3.org/Archives/Public/public-fx/2014AprJun/0141.html

commit d00cc3150b867adde3ac2f65998ed70bae043b6b
Author: Brian Birtles <birtles@gmail.com>
Date:   Thu Sep 25 14:57:05 2014 +0900

    Qualify calculation of directed time when the current iteration is infinity