For minimizing the previous patch changes, `scale`'s default value for
`screenX/Y` is treated as 1.0. It means that `screenX/Y` are device
pixels by default, but `offsetX/Y` are in CSS pixels by default. This
difference may make developers confused. Therefore, we should align the
default unit of them to `screenPixelsPerCSSPixel`. I.e., their default
unit becomes CSS pixels.
Differential Revision: https://phabricator.services.mozilla.com/D105929
