Implement {Worker,DedicatedWorkerGlobalScope}.onmessage.
Merge pull request #3077 from Ms2ger/onmessage

Implement {Worker,DedicatedWorkerGlobalScope}.onmessage; r=abinader+Manishearth
Enable some workers tests.
Disable Worker_terminate_event_queue.htm for its thousands of lines of output.
Update expected results in non-debug builds.
Merge pull request #3079 from Ms2ger/tests-workers-2

Enable some workers tests; r=jgraham
Structured clone the argument to Worker.postMessage.
Structured clone the argument to DedicatedWorkerGlobalScope.postMessage.
Update expectations.