Ensure padding inflation does not cause block scroll in single-line input elements.
These elements can't be scrolled block-direction by user anyway. It can be
programatically moved, but other browsers don't support that (See bug 1933477).
This also preserve our heuristics for finding user-scrollable elements.
Differential Revision: https://phabricator.services.mozilla.com/D230277
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1932840
gecko-commit: 6f0c8239c94d1d285b08727f4c642046f04db740
gecko-reviewers: layout-reviewers, emilio