Auto merge of #15115 - Wafflespeanut:lop, r=emilio

Introduce the `NoCalcLength`

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I began this for making the `CalcLengthOrPercentage` represent `LengthOrPercentage` (instead of the enum we already have), but only later did I realize that it will make `LengthOrPercentageOrFoo` types fatty (which is the problem we're trying to avoid - #15061) and so, I dropped that attempt. Along the way, I introduced an internal type for `Length`, for representing all its non-calc variants (which are `Copy`). We could still have this type for the `LengthOrPercentageOrFoo` types which don't really need  `Length` since they already have their own variants for calc.

r? @Manishearth @emilio @SimonSapin or anyone interested

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [x] These changes do not require tests because it's a refactor

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components/style/gecko/media_queries.rs

@@ -236,8 +236,7 @@ impl MediaExpressionValue {
             nsMediaFeature_ValueType::eLength => {
                 debug_assert!(css_value.mUnit == nsCSSUnit::eCSSUnit_Pixel);
                 let pixels = css_value.float_unchecked();
-                Some(MediaExpressionValue::Length(
-                        specified::Length::Absolute(Au::from_f32_px(pixels))))
+                Some(MediaExpressionValue::Length(specified::Length::from_px(pixels)))
             }
             nsMediaFeature_ValueType::eInteger => {
                 let i = css_value.integer_unchecked();