LibWeb: Take margin box into account when justifying flex items

Before this patch, we were justifying based on the content box only,
which led to misalignments along the main axis when items had non-zero
padding, borders or margins.

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp

@@ -1105,6 +1105,10 @@ void FlexFormattingContext::distribute_any_remaining_free_space()
 
             if (is_main_axis_margin_second_auto(flex_item->box))
                 ++auto_margins;
+
+            used_main_space += flex_item->margins.main_before + flex_item->margins.main_after
+                + flex_item->borders.main_before + flex_item->borders.main_after
+                + flex_item->padding.main_before + flex_item->padding.main_after;
         }
 
         if (flex_line.remaining_free_space > 0) {