Bug 1002332 - Make AnimationTimeline record the last refresh time and use that when there is no refresh driver; r=bz

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.