Implement a stub Worker interface.

Part of #2811.
Implement stub classes for WorkerGlobalScope and DedicatedWorkerGlobalScope.

Part of #2811.
Add worker scope variants to the Global enums.
Parse the URL passed to the Worker constructor.
Spawn a thread for workers.
Add a constructor for DedicatedWorkerGlobalScope.
Load scripts in workers.
Store the JSContext in a field on the worker global scope.
Implement global.console in workers.
Support Worker globals in global_object_for_js_object.
Implement WorkerGlobalScope.self.
Create Worker objects.
Explicitly spawn a native task for workers.
Add a spec link for the Worker constructor.
Merge pull request #2842 from Ms2ger/workers

Basic Web Workers; r=Manishearth,larsbergstrom