[css-scrollbars] Fix animation type of scrollbar-width

See #8873

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -188,7 +188,7 @@ Value: auto | thin | none
 Initial: auto
 Inherited: no
 Computed value: specified keyword
-Animation type: by computed value
+Animation type: discrete
 Applies to: [=scroll containers=]
 Percentages: n/a
 </pre>