This new API allows getting the element which establishes an element's
scroll container. This will be used to properly implement
`scrollIntoView`. There is still work to do for this API and
`offsetParent` to properly handle ancestors which are
closed-shadow-hidden from the original query element.
In addition, fix an issue where inline boxes were establishing scrolling
containers (they shouldn't do that).
Testing: There are tests for this change.
Fixes: #39096.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>