Investigate Window's blur()

[annevk]

At the moment the specification encourages this to be implemented as a no-op. Is there a way we could test this and encourage implementations to adopt this?

From limited testing I could not get all implementations agree to focus a nested window w using w.focus(). That makes it hard to tell if w.blur() does nothing or only does nothing when there's nothing focused.

(Also test blur() after removing the nested window's container element.)

[annevk]

(Relates to #4363.)