This implements the spec change in 21de090dac
The spec change refers to a binary 'animation direction' flag. Instead of that,
however, we just pass the playback rate along and use it inside
GetComputedTimingAt since this seems simpler.
Also, this patch moves the implementation of
KeyframeEffectReadOnly::GetComputedTiming from the header file into the .cpp
file. This is because with this change, GetComputedTiming needs to call
mAnimation->PlaybackRate() and so mozilla::dom::Animation needs to be a complete
type. However, simply including Animation.h doesn't work because of a cyclic
dependency between KeyframeEffect.h and Animation.h. We might be able to fix
this later but since yet-to-land bug 1049975 moves this code around a lot, I'd
rather not touch it too much just now.
MozReview-Commit-ID: 1h6XRh4xmfI
4bf632c00c