fix a race condition when vomnibar iframe is in another process
fix a race condition when vomnibar iframe is in another process

e.g.: press `b` and type `ma` at once,
then Vomnibar.show may not be called,
because a new "update" event resets @onUpdate

gdh1995/vimium-plus,gdh1995/vimium-plus,gdh1995/vimium-plus