[scroll-animations-1] Update ScrollTimeline.start/end types

Author: birtles

scroll-animations-1/Overview.bs

@@ -356,8 +356,8 @@ interface ScrollTimeline : AnimationTimeline {
   constructor(optional ScrollTimelineOptions options = {});
   readonly attribute Element source;
   readonly attribute ScrollDirection orientation;
-  readonly attribute DOMString start;
-  readonly attribute DOMString end;
+  readonly attribute (DOMString or ElementBasedOffset) start;
+  readonly attribute (DOMString or ElementBasedOffset) end;
   readonly attribute (double or ScrollTimelineAutoKeyword) timeRange;
 };
 </pre>