[worker] Integration of WorkerGlobalScope and AbstractWorker

[Bang Seongbeom]

The current spec says that the internal worker scope is 
WorkerGlobalScope(or Dedi-, Shared-), the external is AbstractWorker(or 
Worker, Shared-.) But it destroys consistency with Window object and 
confuses users with the difference of WorkerGlobalScope and AbstractWorker.

The global scope Window object can't access to the other Window object 
that's derived from the other origin because of the Same Origin Policy. 
Like this, we can restrict the access by it, not to separate the two 
objects.

And it's hard to understand that why can't terminate in 
WorkerGlobalScope and can't close in AbstractWorker. These two methods 
should be provided equally, or not.