layout: Basic implementation of size keywords on flex-basis (#35413)

layout: Basic implementation of size keywords on `flex-basis`

This splits the logic to resolve the used value of `flex-basis` into its
own method, which preserves size keywords.

And then it changes `flex_base_size()` to resolve the provided keywords
properly. However, it doesn't handle size keywords in the cross axis.

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

components/layout_2020/sizing.rs

@@ -57,6 +57,13 @@ impl ContentSizes {
             max_content: self.max_content + other.max_content,
         }
     }
+
+    pub fn map(&self, f: impl Fn(Au) -> Au) -> Self {
+        Self {
+            min_content: f(self.min_content),
+            max_content: f(self.max_content),
+        }
+    }
 }
 
 impl Zero for ContentSizes {