servo: Merge #20575 - Use correct filler y value when animating with an implied-y translate()s (from Manishearth:filler-y); r=emilio

r=emilio https://bugzilla.mozilla.org/show_bug.cgi?id=1451724

Source-Repo: https://github.com/servo/servo
Source-Revision: e460b4ab8a9abfe70d0cac9cf03160510f791b7c

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 8d1bb775be1dd91e7b050b867fb41fbdd482005d

servo/components/style/properties/helpers/animated_properties.mako.rs

@@ -1204,7 +1204,8 @@ impl Animate for ComputedTransformOperation {
             ) => {
                 Ok(TransformOperation::Translate(
                     fx.animate(tx, procedure)?,
-                    Some(fy.unwrap_or(*fx).animate(&ty.unwrap_or(*tx), procedure)?)
+                    Some(fy.unwrap_or(LengthOrPercentage::zero())
+                        .animate(&ty.unwrap_or(LengthOrPercentage::zero()), procedure)?)
                 ))
             },
             (