Handle empty sized contents as valid targtes for CSS scroll snap.
Also this change treats edge cases
(e.g. snappedPort.x == target.mSnapArea.XMost()) as valid, I think it would be
better results.
Differential Revision: https://phabricator.services.mozilla.com/D221566
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1914178
gecko-commit: 4901f46fe69df84b27dbfe5f043c4954799a77b8
gecko-reviewers: emilio