This isn't needed as the border box query already takes into account the
containing block chain. Instead, consistently calculate the new
scroll position for a scroller relative to its current scroll offset.
In addition, fix a small bug where the border of a scroll container was
considered part of scrollport.
Testing: A new WPT test is added.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>
