keySystemAccess in MediaCapabilitiesDecodingInfo should be nullable in the dictionary declaration

[alastor0325]

In the MediaCapabilitiesDecodingInfo's webidl declaration, it doesn't declare keySystemAccess as a nullable object. But keySystemAccess should be a nullable object based on other descrptions in the spec, eg.

A MediaCapabilitiesDecodingInfo has associated keySystemAccess which is a MediaKeySystemAccess or null as appropriate.

and 4.1

Set keySystemAccess to null.

[markafoltz]

I am closing this given the commit that was landed.

[markafoltz]

Actually there is an open PR for this (#223), which was incorporated into #107. So this can be closed when #107 is merged.