Bug 1783936 - Do scroll snap without IntendedDirection in the case of non-fling scrolling. r=botond

It matches "a panning gesture, released without momentum" case in "the type of
scrolling methods" [1] section in the spec.

[1] https://drafts.csswg.org/css-scroll-snap/#scroll-types

Differential Revision: https://phabricator.services.mozilla.com/D155049