[web-animations-2] Iteration duration is never set if specified iteration duration is "auto" in "convert a time based animation to a proportional animation"

[Calme1709]

Within the convert a time-based animation to a proportional animation algorithm, the steps associated with the case where (specified) iteration duration is "auto" only tell us to set the start delay and end delay, leaving iteration duration untouched:

Set start delay and end delay to 0, as it is not possible to mix time and proportions.

This doesn't match the behavior of either Chromium or WebKit which respectively set the iteration duration to the intrinsic animation duration and timeline duration / iteration count (which is functionally equivalent to the intrinsic animation duration).