Preserve the unit when interpolating/adding angles with matching units.

If the units of two angles being interpolated/added matches, we should preserve
the original unit; otherwise, we fall back to radians. This matches the behavior
of Gecko.

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

@@ -936,9 +936,20 @@ impl Animatable for i32 {
 impl Animatable for Angle {
     #[inline]
     fn add_weighted(&self, other: &Angle, self_portion: f64, other_portion: f64) -> Result<Self, ()> {
-        self.radians()
-            .add_weighted(&other.radians(), self_portion, other_portion)
-            .map(Angle::from_radians)
+        match (*self, *other) {
+            % for angle_type in [ 'Degree', 'Gradian', 'Turn' ]:
+            (Angle::${angle_type}(val1), Angle::${angle_type}(val2)) => {
+                Ok(Angle::${angle_type}(
+                    try!(val1.add_weighted(&val2, self_portion, other_portion))
+                ))
+            }
+            % endfor
+            _ => {
+                self.radians()
+                    .add_weighted(&other.radians(), self_portion, other_portion)
+                    .map(Angle::from_radians)
+            }
+        }
     }
 }