Wrap `BoxFragment` rare data with `AtomicRefCell` and move
`resolved_sticky_offset` to the `BoxFragmentRareData` since it is
relevant only for `position: sticky`. It reduce the size of
`BoxFragment` from 296 to 264. This allow us to add more rare data to
`BoxFragment` after the reflow, such as the calculation of scrollbar
that requires scrollable overflow to be calculated beforehand.
Testing: No WPT change.
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Signed-off-by: Jo Steven Novaryo <steven.novaryo@gmail.com>
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