Also, in EventStateManager, convert `ComputeScrollTarget()` and
`ComputeScrollTargetAndMayAdjustWheelEvent()` to return `ScrollContainerFrame*`
so that we can drop more unnecessarily `do_QueryFrame`.
Due to removing `nsIScrollableFrame` forward declaration in DOM headers, we have
to add `nsIScrollableFrame` forward declaration temporarily in
nsMenuPopupFrame.h to make this patch compile, but we'll remove it once we
remove `nsIScrollableFrame` under layout/.
Differential Revision: https://phabricator.services.mozilla.com/D211496
