After moving the sandbox IPC font functions off of the sandbox IPC file descriptor, the only thing that is missing is METHOD_MAKE_SHARED_MEMORY_SEGMENT, used by Nacl. 

Once that can be moved to Mojo, we should be able to remove the sandbox IPC filedescriptors from most/all process types. Compare discussion in
https://docs.google.com/a/google.com/document/d/1dLBekS3RlcQFldeYZlzFGkuLyX53GBr_7Hbf4cUz1OE/edit?disco=AAAAB8uIPNs