Instead of storing the non-pseudo version of the node in
`NodeAndStyleInfo`, store the pseudo version and use that to query the
`PseudoElement` that a `NodeAndStyleInfo` refers to.
This is a step on the way toward fixing nested pseudo-elements in Servo.
Testing: This should not change behavior and is thus covered by existing
WPT tests.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>