Fix race condition in grab-back-focus.
Fix race condition in grab-back-focus.

See the newly-added comment.

decaffeinate-examples/vimium,smblott-github/vimium,philc/vimium,poacher2k/vimium,willsALMANJ/vimium,philc/vimium,mrmr1993/vimium,gdh1995/vimium,mrmr1993/vimium,gdh1995/vimium,poacher2k/vimium,smblott-github/vimium,mrmr1993/vimium,decaffeinate-examples/vimium,philc/vimium,poacher2k/vimium,willsALMANJ/vimium,willsALMANJ/vimium,gdh1995/vimium,decaffeinate-examples/vimium,smblott-github/vimium