The Windows bot is usually slower than the Linux bot, and therefore
text layer rendering is as well. However, the `autoprint` test awaited
text layer rendering to complete before activating the selector check,
which makes it timing-sensitive and causes it to never resolve because
the page is already printed (and the printed page div removed) by then.
This commit should fix the issue by activating the selector check as
soon as possible, namely as soon as the viewer appears, which should
ensure we're always registering the selector check in time because we're
doing it even before rendering is starting.
2c1d95b153