This requires quite a bit of piping to get the ChildID passed everywhere where
we currently pass the pid in IPC. This is done by adding a new struct type
(EndpointProcInfo), which is passed around instead of OtherPid in these places,
and contains the full pid.
In most cases, it was a fairly painless change to move over, however in some
cases, more complex changes were required, as the pid was being stored
previously in something like an Atomic<...>, and needed to be switched to using
a mutex-protected value.
In the future, it may be possible to remove OtherPid from IPDL actors once
everything is migrated to ChildID, but we're still a long way off from that, so
for now we unfortunately need to pass both around.
Differential Revision: https://phabricator.services.mozilla.com/D217118
2c721f79d8