onMessage top-level property

An event handler fired whenever a message event occurs — when incoming messages are received. Controlled pages can use the MessagePort.postMessage() method to send messages to service workers. The service worker can optionally send a response back via the MessagePort exposed in event.data.port, corresponding to the controlled page. onmessage is actually fired with ExtendableMessageEvent, but since we are merging the interface into Window, we should make sure it's compatible with window.onmessage onmessage: (messageevent: ExtendableMessageEvent) => void;