Crashing Gecko in register_handler

[kvark]

Issue created from servo/webrender#3322 (comment) discussion.

I wonder if Gecko tries to create 2 different Renderer instances on the same (compositor) thread. We should also make sure to clean up after ourselves by adding the exit_handler that would "unregister" the profiler.

[atouchet]

@gw3583 do you still have access to this repo?