We have already resolved to make calling Finish() clear the pause state (see
https://lists.w3.org/Archives/Public/public-fx/2015AprJun/0038.html, item 2).
Doing that involves resolving the start time when the animation is paused.
Furthermore, as a separate change, we resolved to make the finished promise not
resolve when the animation is paused. That suggests making UpdateFinishedState()
only resolve the finished promise when PlayState() == Finished rather than using
IsFinished() which returns true even if the animation is paused.
However, if we compare PlayState() == Finished in UpdateFinishedState() then we
will *not* resolve the finished promise when the animation is play-pending since
PlayState() == Pending in that case (pause-pending is ok since the call to
SetCurrentTime will cause a transition to the Paused state). Furthermore, the
existing call to cancel the pending play task will effectively leave this
animation forever pending. Hence, in this patch we unconditionally fill in the
start time.
