Bug 1080584 - Part 1: Clean up ArenaLists::allocateFromArena some more. r=terrence

--HG--
extra : rebase_source : 1b557eba3399b1869aec673582e3bc3952bbad2e
Bug 1080584 - Part 2: Remove BFS_JUST_FINISHED since it doesn't really add any protection. r=terrence

--HG--
extra : rebase_source : e4d747239f7c7d9075738c1acd75331127978ebd