Auto merge of #14049 - Wafflespeanut:lon, r=SimonSapin

Add a generic type for sharing some CSS types

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This would be useful for types such as `T(pub Option<U>)` or any type that's a mixture of two types (like `LengthOrFoo` for example). Well, it's a bit ugly, especially because we have to address the types as `Either::First` and `Either::Second` everywhere, but I don't have a brighter idea 😄

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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

r? @SimonSapin (cc @Manishearth @emilio)

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components/layout/fragment.rs

@@ -49,7 +49,8 @@ use style::properties::ServoComputedValues;
 use style::selector_impl::RestyleDamage;
 use style::servo::restyle_damage::RECONSTRUCT_FLOW;
 use style::str::char_is_whitespace;
-use style::values::computed::{LengthOrNone, LengthOrPercentage, LengthOrPercentageOrAuto};
+use style::values::Either;
+use style::values::computed::{LengthOrPercentage, LengthOrPercentageOrAuto};
 use style::values::computed::LengthOrPercentageOrNone;
 use text;
 use text::TextRunScanner;
@@ -2586,7 +2587,7 @@ impl Fragment {
 
         // TODO(mrobinson): Determine if this is necessary, since blocks with
         // transformations already create stacking contexts.
-        if self.style().get_effects().perspective != LengthOrNone::None {
+        if let Either::First(ref _length) = self.style().get_effects().perspective {
             return true
         }