These assertions don't hold in some perfectly legitimate cases, such as when
items have both top and bottom sticky behaviours. The actual behaviour still
seems ok, so let's just drop the assertions.

Differential Revision: https://phabricator.services.mozilla.com/D70459

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/259c147efa83074869ce93c3053b4ecbd80863d6