Animation::ResumeAt contains an assertion that, when we exit the play-pending
state, checks we either have a resolved start time or a resolved hold time.
That's normally true but if we are aborting a pause on animation that is
finished we can end up with a resolved start time (since we don't clear the
start time when we're aborting a pause) and a resolved hold time (either
because the regular finishing behavior set one, or, because play() applied
auto-rewinding behavior and set it).
In that case we should actually respect the hold time and update the start time
when resuming the animation. However, ResumeAt won't update the start time if it
is already set.
This patch fixes that by clearing the start time in DoPlay in the case where we
are aborting a pause and have a hold time.
