Rename updateIterationDuration() to _updateInternalState()
Rename updateIterationDuration() to _updateInternalState()

This is a more appropriate name now that all duration calculations are done
lazily, Also remove a couple of stale comments.

web-animations/web-animations-js-legacy,dstockwell/web-animations-js,web-animations/web-animations-js-legacy,dstockwell/web-animations-js,beni55/web-animations-js-legacy,beni55/web-animations-js-legacy,birtles/web-animations-js,birtles/web-animations-js,web-animations/web-animations-js-legacy,beni55/web-animations-js-legacy,beni55/web-animations-js-legacy,birtles/web-animations-js