LibWeb: Improve FFC step 7 (hypothetical cross size)

- Avoid performing inside layout on definite-size flex items (since
  their computed size can be used as-is.)

- Use FormattingState::clone() to generate a throwaway layout instead of
  mutating the tree in-place.

- Update spec link & comments based on current CSSWG draft. The latest
  version is quite a bit clearer on how this should work.

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.h

@@ -96,7 +96,7 @@ private:
 
     void resolve_flexible_lengths(float main_available_size);
 
-    float determine_hypothetical_cross_size_of_item(Box const&);
+    void determine_hypothetical_cross_size_of_item(FlexItem&);
 
     void calculate_cross_size_of_each_flex_line(float cross_min_size, float cross_max_size);