Add a test for the event dispatched by DedicatedWorkerGlobalScope.postMessage.
Merge pull request #1213 from Ms2ger/DWGS-postMessage-event

Add a test for the event dispatched by DedicatedWorkerGlobalScope.postMessage; r=zcorpan
Rewrite postMessage_event_properties to make it more robust.
Remove trailing whitespace.
Add additional assertions to postMessage_event_properties.htm.