style: Part 2: Support fit-content() in style

Support fit-content for preferred size, min size, and max size. This
patch only implement the style system. For layout part, we will do that
in the following patches.

Differential Revision: https://phabricator.services.mozilla.com/D107161

components/style/values/computed/length.rs

@@ -188,7 +188,8 @@ impl Size {
             GenericSize::MinContent |
             GenericSize::MaxContent |
             GenericSize::MozFitContent |
-            GenericSize::MozAvailable => false
+            GenericSize::MozAvailable |
+            GenericSize::FitContentFunction(_) => false
         }
     }
 }