servo: Merge #18069 - Make scroll-snap-{coordinate, destination} discrete animation (from hiikezoe:scroll-snap); r=daisuke

<!-- Please describe your changes on the following line: -->
https://bugzilla.mozilla.org/show_bug.cgi?id=1387941

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

Source-Repo: https://github.com/servo/servo
Source-Revision: 421a553799b6e71c2bf7a0f5bba4082060445533

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 1ade80b1376d7e773397564d48b88a99549628fd

servo/components/style/properties/longhand/box.mako.rs

@@ -721,7 +721,7 @@ ${helpers.predefined_type("scroll-snap-destination",
                           products="gecko",
                           boxed="True",
                           spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/scroll-snap-destination)",
-                          animation_value_type="ComputedValue")}
+                          animation_value_type="discrete")}
 
 ${helpers.predefined_type(
     "scroll-snap-coordinate",
@@ -730,7 +730,7 @@ ${helpers.predefined_type(
     vector=True,
     products="gecko",
     spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/scroll-snap-destination)",
-    animation_value_type="ComputedValue",
+    animation_value_type="discrete",
     allow_empty="NotInitial"
 )}