style: Three-value position syntax uses calc() as its computed value representation.

This restores the previous behavior of using calc().

Note that background-position / object-position, which test this, weren't
hitting the assertion because they use another codepath.

I didn't add more extensive tests for this because it's well tested for those
two properties, and because this is legacy anyway, see the comment in the test.

I did add the assertion to the codepath those two properties hit.

Differential Revision: https://phabricator.services.mozilla.com/D16176

components/style/values/specified/position.rs

@@ -263,11 +263,12 @@ impl<S: Side> ToComputedValue for PositionComponent<S> {
                 let length = length.to_computed_value(context);
                 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,
-                    length.was_calc,
+                    /* was_calc = */ true,
                 )
             },
             PositionComponent::Side(_, Some(ref length)) |