This feature should not be shipped until the various definitions of addition for
each additive property are properly specified.
Unlike other patches in this series, compositing is not frequently used
internally (e.g. by DevTools etc.) so there is no need to enable this by default
for system code.
Also, it turns out we have inadvertently been shipping part of this feature for
some time now. The next patch in this series will add tests for that case and
disable that part of the feature (a suitable intent to unship will follow). This
patch merely adapts and extends the existing tests without affecting the surface
area covered by the combination of the newly-added pref and the existing
dom.animations-api.core.enabled pref.
MozReview-Commit-ID: Htr6mlyCBav
--HG--
rename : dom/animation/test/mozilla/file_disable_animations_api_core.html => dom/animation/test/mozilla/file_disable_animations_api_compositing.html
rename : dom/animation/test/mozilla/test_disable_animations_api_core.html => dom/animation/test/mozilla/test_disable_animations_api_compositing.html
extra : rebase_source : 7715a25e59568eb122ba3f7dbd2c2b2ffdd19954
