Bug #20845: Make the Circuit Display runtime togglable (Default: Off)

My tinfoil hat went *crinkle* after the recent SVG/JS fun.

Until e10s is part of Tor Browser, the firefox process in all but the
most casual threat models has zero business in knowing what the user's
guard is.

The Circuit Display (and the associated surrogate control port commands)
now must explciitly be enabled like PulseAudio.