Add Tor Browser indicator where Firefox PB indicator is
We need to keep Tor Browser looking distinct from other browsers. One way is to show an indicator the way Firefox has a "private browsing indicator" (an eyemask on a purple background, in a circular icon):
The PB icon file (24x24 svg) is here: https://dxr.mozilla.org/mozilla-central/raw-rev/085cdfb90903d4985f0de1dc7786522d9fb45596/browser/themes/shared/icons/private-browsing.svg
We'll need a new icon to fit there. I would suggest using a different color so it's easy to distinguish from Firefox PBM even in the user's peripheral vision.