There are no callers from JS (looked at the selection region constant
usage in idl and those use nsISelectionController, not this).
If we were to provide this API in the future, it would probably use
ScrollIntoViewOptions or so instead (plus something extra perhaps).
I wanted to remove the weird WehreToScroll percentage impl, but actually
some C++ accessibility code still uses it.
Differential Revision: https://phabricator.services.mozilla.com/D216872