Update to constructor operations

[ChunMinChang]

[Constructor(optional MediaMetadataInit init), Exposed=Window]
interface MediaMetadata {
  ...
};

should be replaced by

[Exposed=Window]
interface MediaMetadata {
  constructor(optional MediaMetadataInit init);
  ...
};

since the syntax is changed in whatwg/webidl#778.

[ChunMinChang]

Not sure if the editors are working on this or not. I can submit a request if no one is working on this.

[ChunMinChang]

Fixed in #236