This change makes it so that the euclid types returned from layout
queries use the `CSSPixel` unit type when appropriate. The minimal set
of changes are also made to avoid having to convert these types to the
`UnknownUnit` in other places. There is still some casting that has to
happen to deal with the difference between Stylo's `CSSPixel` and
WebRender's `LayoutPixel`, but a followup changes will try to switch to
using one or the other.
Testing: This should not change behavior, so is covered by existing
tests.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>