Auto merge of #17906 - emilio:calc-bgsize, r=hiro,birtles

style: Don't unconditionally generate Calc values when converting background-size from Gecko.

Bug: 1385140
MozReview-Commit-ID: ERwq50WSvLV

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17906)
<!-- Reviewable:end -->

components/style/properties/gecko.mako.rs

@@ -3446,7 +3446,7 @@ fn static_assert() {
                 LengthOrPercentageOrAuto::Auto
             } else {
                 debug_assert!(ty == DimensionType::eLengthPercentage as u8);
-                LengthOrPercentageOrAuto::Calc(value.into())
+                value.into()
             }
         }