tests/chromium: Work around popup close flakiness.

It's not nice to send the escape key over and over again just to ensure
the popup is closed, because even *if* it fails to close the popup 4
times in a row it's just very unlikely that it will be closed. But in
order to make really sure, we might need to do a screenshot and detect
visual changes.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>