This is probably the last thing we will ship (if ever) since it needs the most
spec and implementation work for arbitrary use that is pretty far into a corner.
This feature should not be shipped until the various definitions of addition for
each additive property are properly specified and then implemented accordingly.
Unlike other patches in this series, compositing is not frequently used
internally so there is no need to enable this by default for chrome callers.
This preference controls whether authors are allowed to specify animations
without a 0% or 100% keyframe.
We intend to ship this but it isn't implemented yet (needs a follow-up) but this
preference acts as a safeguard in case we discover we need to disable it once
it's implemented.
