When calling a Recv/Alloc/Dealloc method on most types, cast `this` to the
derived class.
There is a heuristic to figure out what the correct derived type is. There is a
blacklist of types which we can't do direct calls on for the moment, as well as
an override for types that do work with direct calls but which don't match the
heuristic.
Differential Revision: https://phabricator.services.mozilla.com/D16492
--HG--
extra : moz-landing-system : lando
