Replace Au with CSSPixelLength in CalcLengthOrPercentage.

We replace Au with CSSPixelLength for the length part of
computed::CalcLengthOrPercentage. Therefore, it would be easier to use
CSSPixelLength for all other LengthOrPercentage{*} types.

components/layout/fragment.rs

@@ -1520,7 +1520,7 @@ impl Fragment {
                     LengthOrPercentageOrAuto::Calc(calc) => {
                         // TODO(nox): This is probably wrong, because it accounts neither for
                         // clamping (not sure if necessary here) nor percentage.
-                        calc.unclamped_length()
+                        Au::from(calc.unclamped_length())
                     },
                 };