As part of supporting arbitrary timelines, we'd like to pass null times to the
function that calculates computed timing. Incidentally, this also provides
a means for evaluating calculating timing parameters that are independent of the
current time (currently only the active duration) without requiring a valid
time.
This patch updates the signature of ElementAnimation::GetComputedTimingAt to
take a nullable time duration.
We use the Nullable wrapper to represent null TimeDurations since, unlike,
TimeStamp, TimeDuration does not include a null state.
3d5b51ab69