We will use the new type for the generated IPDL message handler
prototype to make sure correct error handling method is called.
MozReview-Commit-ID: AzVbApxFGZ0
The only thing we needed opened actor tracking for was the ability to
clone all the actors. But now that we no longer have support for
cloning actors, we no longer need to track the actors that we've cloned,
which makes a number of things significantly simpler.
