LibWeb: Add auto as a recognized argument of flex-basis

There isn't actually any special treatment of this over 'content' in
the FlexFormattingContext, for now both are treated the same.
This fixes #9225

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -96,7 +96,8 @@ enum class FlexWrap {
 
 enum class FlexBasis {
     Content,
-    Length
+    Length,
+    Auto,
 };
 
 enum class WhiteSpace {