layout: Support min/max main keyword sizes in flexbox (#35961)

Adds support for min-content, max-content, fit-content and stretch on
the min and max main size properties of a flex item.

I'm removing `automatic_min_size()` and `flex_base_size()` because they
would need to share so much code among themselves and their one caller
that it's simpler to just inline the code.

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

components/layout_2020/geom.rs

@@ -729,12 +729,6 @@ impl<T: Clone> Size<T> {
         }
     }
 
-    #[inline]
-    pub(crate) fn to_auto_or(&self) -> AutoOr<T> {
-        self.to_numeric()
-            .map_or(AutoOr::Auto, AutoOr::LengthPercentage)
-    }
-
     #[inline]
     pub fn map<U>(&self, f: impl FnOnce(T) -> U) -> Size<U> {
         match self {