[web-animations-1] Specify accumulation of transform lists more precisely

[birtles]

From @birtles on September 30, 2015 4:48

https://w3c.github.io/web-animations/#issue-3540bd4c
https://w3c.github.io/web-animations/#issue-7c98bbea

Copied from original issue: w3c/web-animations#114

[birtles]

Accumulation of transform lists is now defined in CSS Transforms Level 2.