Bug 972199 part 1 - Make SampleAnimations filter out animations that are yet to start; r=nrc

When restoring the refresh driver after testing we can arrive at a situation
where a layer has an animation that has yet to start. However, calling
ElementAnimations::GetPositionInIteration with a negative value from the
compositor thread is an error.

This patch detects animations on the compositor thread that are yet-to-start and
skips them when sampling. It also moves the activeAnimations = true line up as
otherwise we would need special logic to wake up the compositor after the delay
has finished.
Bug 972199 part 2 - Add a crashtest for restoring the refresh driver after an advance; r=nrc

In this crash test we restore the refresh driver after manually advancing it.
This causes a situation where a layer has an animation that has yet to start.

Prior to modifying ElementAnimations::GetPositionIteration this test case would
trip an assertion there that rejected negative elapsed times when called from
the compositor thread.
Bug 972199 part 3 - Fix comment in ElementAnimations::GetPositionInIteration; r=nrc

Make comment reflect current parameter names.