Round halfway values toward positive infinity for integer type of ani…

…mation.

From the spec[1];

 `with values halfway between a pair of integers rounded towards positive infinity.`

[1] https://drafts.csswg.org/css-transitions/#animtype-integer

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

@@ -863,7 +863,7 @@ impl Animatable for f64 {
 impl Animatable for i32 {
     #[inline]
     fn add_weighted(&self, other: &i32, self_portion: f64, other_portion: f64) -> Result<Self, ()> {
-        Ok((*self as f64 * self_portion + *other as f64 * other_portion).round() as i32)
+        Ok((*self as f64 * self_portion + *other as f64 * other_portion + 0.5).floor() as i32)
     }
 }