Adapt api.MediaKeySession.onmessage to new events structure (#15167)
Adapt api.MediaKeySession.onmessage to new events structure (#15167)

This PR adapts the message event of the MediaKeySession API to conform to the new events structure.
mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data