restore "black on black constrast fix" to ESR45
brade and mcs's patch to prevent exposing system colors to CSS and canvas was partially upstreamed, but there is another part that needs to be rebased to ESR45. Because this task is somewhat more complicated, I am creating this ticket.
It would also be good to have a unit test under Tor Project's control that ensures that the upstreamed part and the non-upstreamed part are providing complete protection.
The original patch in TBB/ESR38 is 8b3d00b982db7a1b3101dc9318c1301bf944b161
The part upstreamed to mozilla (and now in ESR45) is 61c6a3b38353835315f2d1b9761de9c95ba83c8d