[web-animations-1] Consider allowing seeking currentTime from resolved to unresolved

[flackr]

web-animations section 4.4.5 of the spec says that:

1. If seek time is an unresolved time value, then perform the following steps.
   1. If the current time is resolved, then throw a TypeError

In the ScrollTimeline spec, scroll-animations section 3.1.4 the current time can go from resolved to unresolved if the ScrollTimeline does not have fill: both. For the ScrollTimeline polyfill, I implemented this transition by canceling the animation

    if (currentTime == null) {
      if (animations[i].playState == 'paused')
        animations[i].cancel();
    } else {
      animations[i].currentTime = currentTime;
    }

However, if transitioning from resolved to unresolved is expected behavior, I think that we should allow seeking to unresolved time values. Of course, doing so may be equivalent to canceling the animation?