style: Split LengthPercentage again.

This is needed to support min() / max() / clamp(), etc, as those need to be a
tree of values and thus need heap storage.

This unfortunately grows LengthPercentage to be two pointers, which is bad as
it blows up the size of nsStylePosition enough to trigger the size assertions.

This patch comments out the assertion for now, the follow-up patches will
uncomment them.

Differential Revision: https://phabricator.services.mozilla.com/D58700

components/style/values/specified/position.rs

@@ -297,7 +297,7 @@ impl<S: Side> ToComputedValue for PositionComponent<S> {
                 let p = Percentage(1. - length.percentage());
                 let l = -length.unclamped_length();
                 // We represent `<end-side> <length>` as `calc(100% - <length>)`.
-                ComputedLengthPercentage::with_clamping_mode(l, Some(p), length.clamping_mode)
+                ComputedLengthPercentage::new_calc(l, p)
             },
             PositionComponent::Side(_, Some(ref length)) |
             PositionComponent::Length(ref length) => length.to_computed_value(context),