Add dictionary default value

Author: saschanaz

Overview.bs

@@ -362,7 +362,7 @@ dictionary ScrollTimelineOptions {
 };
 
 [Exposed=Window,
- Constructor(optional ScrollTimelineOptions options)]
+ Constructor(optional ScrollTimelineOptions options = {})]
 interface ScrollTimeline : AnimationTimeline {
   readonly attribute Element scrollSource;
   readonly attribute ScrollDirection orientation;