That is the issue; unless the fire trap, or source of fire damage, makes an attack roll as part of the of the "start of the turn" portion of a turn, then it is not technically an attack. The Troll Healing power specifically calls out that it is an attack. RAW, the troll lives.
I do agree though, I would rule that the troll is cooked once it takes fire damage at 0 hp (unless the nature of the battle requires that the troll be perniciously difficult to kill).
