Since bug 1922904, GetSnapPointForDestination had returned an ineligible
snap point.
For exmaple, given that there's a scroll container only wants to snap
horizontally and there are 3 snap target points, (0, 0), (100, 0) and
(200,0). Then if the user tries to scroll vertically to (0, 100), the
function returned (0, 0) as a valid snap position for the vertical scroll
operation. With this change the function now returns (0, 100) so that
the user can scroll vertically to the correct position.
Differential Revision: https://phabricator.services.mozilla.com/D231058
51fc6df674