Tweak CalcLengthOrPercentage to use pixel value.

We compute the distance for eCSSUnit_Calc by pixel value in Gecko,
so let's follow the same rules.

components/style/values/computed/length.rs

@@ -78,7 +78,8 @@ impl ComputeSquaredDistance for CalcLengthOrPercentage {
         // FIXME(nox): This looks incorrect to me, to add a distance between lengths
         // with a distance between percentages.
         Ok(
-            self.unclamped_length().compute_squared_distance(&other.unclamped_length())? +
+            self.unclamped_length().to_f64_px().compute_squared_distance(
+                &other.unclamped_length().to_f64_px())? +
             self.percentage().compute_squared_distance(&other.percentage())?,
         )
     }