Adapt api.MediaKeySession.onmessage to new events structure

[queengooborg]

This PR adapts the message event of the MediaKeySession API to conform to the new events structure.

[foolip]

https://source.chromium.org/chromium/chromium/src/+/6e3647df59a9b8a2c77d49ce5681e2a5b3ab7d17 shows that the event handler attributes were added after the events themselves. Can you add partial_implementation ranges?

[Elchi3]

I think the following is needed here:

"chrome": [
  {
    "version_added": "55"
  },
  {
    "version_added": "42",
    "version_removed": "55",
    "partial_implementation": true,
    "notes": "The <code>onkeystatuseschange</code> event handler property is not supported."
  }
],

And

"chrome": [
  {
    "version_added": "55"
  },
  {
    "version_added": "42",
    "version_removed": "55",
    "partial_implementation": true,
    "notes": "The <code>onmessage</code> event handler property is not supported."
  }
],

(same structure [with different versions] for other chromium based browsers)

[foolip]

Confirmed by https://storage.googleapis.com/chromium-find-releases-static/6e3.html#6e3647df59a9b8a2c77d49ce5681e2a5b3ab7d17