LibWeb: Fix flex line alignment in single-line flex containers

For single-line flex containers, center the only flex line along the
cross axis. Alignment of multi-line flex containers are left as a FIXME.

This patch also moves out the assignment of final metrics to the
FormattingState from align_all_flex_lines() to a separate function.

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.h

@@ -81,6 +81,7 @@ private:
     void set_main_axis_second_margin(Box const&, float margin);
 
     void setup_initial_width_and_height();
+    void copy_dimensions_from_flex_items_to_boxes();
 
     void generate_anonymous_flex_items();