[web-animations-1] Add CEReactions to commitStyles

Closes #7507

Author: birtles

web-animations-1/Overview.bs

@@ -4129,6 +4129,7 @@ interface Animation : EventTarget {
     undefined updatePlaybackRate(double playbackRate);
     undefined reverse();
     undefined persist();
+    [CEReactions]
     undefined commitStyles();
 };
 </pre>