style: Explicitly specify animation_value_type for scroll-padding.

Otherwise the Trait for clamping negative animation value isn't generated thus
negative animating results are exposed in computed values.

Differential Revision: https://phabricator.services.mozilla.com/D21153

components/style/properties/longhands/padding.mako.rs

@@ -39,6 +39,6 @@
         logical=side[1],
         logical_group="scroll-padding",
         spec="https://drafts.csswg.org/css-scroll-snap-1/#propdef-scroll-padding-%s" % side[0],
-        animation_value_type="ComputedValue",
+        animation_value_type="NonNegativeLengthPercentageOrAuto",
     )}
 % endfor