Auto merge of #14740 - glasserc:extremum-length, r=Manishearth

Add support for keyword values for min-width and max-width

This is a follow-up to #14432 which got closed and can no longer be re-opened.

This PR aims to add support for keyword-values max-content, min-content, fit-content, and fill-available to the properties that use them, namely min-width, min-height, max-width, and max-height.

It's still untested because I still haven't figured out how to do that. I guess I should write or find some web page that uses these properties.

Refs #13821.

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- [ ] `./mach build -d` does not report any errors
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- [ ] These changes fix #13821  (github issue number if applicable).

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components/style/values/mod.rs

@@ -185,3 +185,11 @@ impl<A: ToComputedValue, B: ToComputedValue> ToComputedValue for Either<A, B> {
         }
     }
 }
+
+// A type for possible values for min- and max- flavors of width,
+// height, block-size, and inline-size.
+define_css_keyword_enum!(ExtremumLength:
+                         "max-content" => MaxContent,
+                         "min-content" => MinContent,
+                         "fit-content" => FitContent,
+                         "fill-available" => FillAvailable);