Stylo: Make border-spacing animatable

Two things are included in this patch:

1. Implement ComputeDistance for border-spacing, so we could get the right
distance while doing animations.

2. Implement clone function for gecko glue code of border-spacing, so we
could make animations of border-spacing work properly in stylo build.

Gecko side patch: Bug 1354437

components/style/properties/gecko.mako.rs

@@ -3255,6 +3255,13 @@ fn static_assert() {
         self.gecko.mBorderSpacingCol = other.gecko.mBorderSpacingCol;
         self.gecko.mBorderSpacingRow = other.gecko.mBorderSpacingRow;
     }
+
+    pub fn clone_border_spacing(&self) -> longhands::border_spacing::computed_value::T {
+        longhands::border_spacing::computed_value::T {
+            horizontal: Au(self.gecko.mBorderSpacingCol),
+            vertical: Au(self.gecko.mBorderSpacingRow)
+        }
+    }
 </%self:impl_trait>
 
 

components/style/properties/longhand/inherited_table.mako.rs

@@ -20,7 +20,7 @@ ${helpers.single_keyword("caption-side", "top bottom",
                          animation_value_type="none",
                          spec="https://drafts.csswg.org/css-tables/#propdef-caption-side")}
 
-<%helpers:longhand name="border-spacing" animation_value_type="none" boxed="True"
+<%helpers:longhand name="border-spacing" animation_value_type="ComputedValue" boxed="True"
                    spec="https://drafts.csswg.org/css-tables/#propdef-border-spacing">
     use app_units::Au;
     use std::fmt;
@@ -29,7 +29,7 @@ ${helpers.single_keyword("caption-side", "top bottom",
 
     pub mod computed_value {
         use app_units::Au;
-        use properties::animated_properties::Interpolate;
+        use properties::animated_properties::{ComputeDistance, Interpolate};
 
         #[derive(Clone, Copy, Debug, PartialEq)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
@@ -48,6 +48,19 @@ ${helpers.single_keyword("caption-side", "top bottom",
                 })
             }
         }
+
+        impl ComputeDistance for T {
+            #[inline]
+            fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
+                self.compute_squared_distance(other).map(|sd| sd.sqrt())
+            }
+
+            #[inline]
+            fn compute_squared_distance(&self, other: &Self) -> Result<f64, ()> {
+                Ok(try!(self.horizontal.compute_squared_distance(&other.horizontal)) +
+                   try!(self.vertical.compute_squared_distance(&other.vertical)))
+            }
+        }
     }
 
     impl HasViewportPercentage for SpecifiedValue {