Define the behavior of MediaKeyStatusMap::get(keyId) when |keyId| is not in the map

[ddorwin]

The definition of MediaKeyStatusMap's get(BufferSource keyId) method is currently:

Returns the MediaKeyStatus of the key identified by keyId.

It does not say what happens or is returned if keyId is not in the map and thus there is no value for it.

To be consistent with the ES6 Map interface's get(), this method should return undefined.

[mwatson2]

I assume the method should return undefined in the same circumstances as the has method returns false.