move to partial interface

Author: stephenmcgruer

web-animations-2/Overview.bs

@@ -1518,10 +1518,7 @@ partial interface AnimationTimeline {
 The Animation interface is amended to make the timeline member mutable:
 
 
-[Exposed=Window,
- Constructor(optional AnimationEffect? effect = null,
-             optional AnimationTimeline? timeline)]
-interface Animation : EventTarget {
+partial interface Animation {
                attribute AnimationTimeline?       timeline;
 };