The spec requires the MediaKeyStatusMap.get(keyId) function to return an 'any'
type, which is undefined for known keys, or a MediaKeyStatus enum value for
known keys.
https://w3c.github.io/encrypted-media/#idl-def-mediakeystatusmap
MozReview-Commit-ID: 3TOFYLacZSc
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extra : rebase_source : cd067f545cdbd9352ba64fea914128201b458d9c