[issues/9490] \OC::$WEBROOT is invalid when called from cron.php

Fix #9490, owncloud/activity#87

@icewind1991 you added the hack into lib/private/request.php 12f7cb87679453fa96c796df857b7e7193d4ee09
@PVince81 @DeepDiver1975 @MorrisJobke @karlitschek @butonic @LukasReschke