Merge pull request #3076 from Ms2ger/DWGS-postMessage

Implement DedicatedWorkerGlobalScope.postMessage; r=Manishearth
Implement {Worker,DedicatedWorkerGlobalScope}.onmessage.
Enable some workers tests.
Structured clone the argument to Worker.postMessage.
Structured clone the argument to DedicatedWorkerGlobalScope.postMessage.