Maintain portsForTab mapping tabIds to frame ports.
Maintain portsForTab mapping tabIds to frame ports.

This maintains a mapping from tab Ids to a mapping from frame Ids to
their ports.

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