Bug 1249274 - Need preferences for reftest using Web Animation APIs. r=birtles

MozReview-Commit-ID: 4vd2sVHx4fQ

--HG--
extra : transplant_source : %E7%05A%0Fk%D0%87JA%1AX%FB%A9%60%83%14Z%D4DP

layout/reftests/web-animations/reftest.list

@@ -1 +1 @@
-== 1246046-1.html green-box.html
+test-pref(dom.animations-api.core.enabled,true) == 1246046-1.html green-box.html