A document that belongs to an iframe that is display:none as no associated pres
context from which to get a refresh driver. However, in this case
document.timeline.currentTime should still never go backwards (since document
timelines are supposed to be monotonically increasing).
This patch makes AnimationTimeline record the last value returned for the
current time so that if the document becomes display:none we can still return
the most recently used time.
This patch simply factors out the conversion from a TimeStamp value to
a nullable-double value relative to the start of the timeline. This is so that,
in a subsequent patch, this functionality can be reused by ElementAnimation
when it reports its start time (which is currently recorded as a TimeStamp).
AnimationTimeline::GetCurrentTime returns the time value of the timeline as
a double. For internal calculations however it is more useful to get this as
a mozilla::TimeStamp.
This patch splits the calculation of the current time into two stages:
calculation as a timestamp then conversion to a double.
