[scroll-animations] Ignore @scroll-timeline rules in shadow trees

For now, that is.

Fixes #5167.

Author: Anders Hartvoll Ruud

scroll-animations-1/Overview.bs

@@ -759,6 +759,11 @@ Once specified, a scroll timeline may be associated with a CSS Animation
 The <<declaration-list>> inside of ''@scroll-timeline'' rule can only contain the
 descriptors defined in this section.
 
+An ''@scroll-timeline'' rule is invalid if it occurs in a stylesheet inside of a
+[=shadow tree=], and must be ignored.
+
+Issue(5167): This will likely change in the future.
+
 ### Scroll Timeline descriptors ### {#scroll-timeline-descriptors}