layout: Add BoxFragment rare data (#38325)

Introduce `BoxFragmentRareData`, rare data for `BoxFragment`, which
would store the specific data that is relevant to several fragments.
This would reduce the `BoxFragment` size to 256 from 264 and add 8 bytes
for fragment that have rare data (due to the additional pointer to the
rare data).

Testing: Existing WPT coverage

Signed-off-by: Jo Steven Novaryo <jo.steven.novaryo@huawei.com>

components/layout/display_list/stacking_context.rs

@@ -1334,7 +1334,7 @@ impl BoxFragment {
         }
 
         if matches!(&fragment, Fragment::Box(box_fragment) if matches!(
-            box_fragment.borrow().specific_layout_info,
+            box_fragment.borrow().specific_layout_info(),
             Some(SpecificLayoutInfo::TableGridWithCollapsedBorders(_))
         )) {
             stacking_context