worker: fix heap snapshot crash on exit
Worker.parent_port_ can be released before the exit event of Worker. The parent_port_ is not owned by node::worker::Worker, thus the reference to it is not always valid, and accessing it at exit crashes the
process.
As the Worker.parent_port_ is only used in the memory info tracking, and the corresponding MessagePort
object is already been tracking with the Worker.messagePort javascript property , it can be safely removed.