"4.7.2 Dedicated workers and the Worker interface
This constructor must be visible when the script's global object is either a Window object or an object implementing the WorkerUtils interface."
It's only visible when the global object is Window so it's not possible to create nested workers.
If you have a use case for nested workers, please comment in bug 22723. Currently, they are not being actively worked on, mainly because there doesn't seem to be any use for them, and implementation cost is rather high.
*** This bug has been marked as a duplicate of bug 25212 ***