Steps 12 and 13 of
https://html.spec.whatwg.org/multipage/browsing-the-web.html#navigate
contains logic to update the provided history handling mode when
navigating, but there is no else clause in the specification. This
change removes the logic in Servo that falls back to "push" mode if
neither step is executed.
Testing: New WPT test added.
Fixes: #39818
Signed-off-by: Josh Matthews <josh@joshmatthews.net>