CueUpdateEvent
public class CueUpdateEvent : NSObject, CueEventProtocolFired when TextTrackCueEventTypes.UPDATE occurs for a TextTrackCue.
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                  The textual representation of the type of event, “update”. DeclarationSwift public let type: String
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                  The date at which the event occurred. DeclarationSwift public let date: Date
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                  The TextTrackCuewhich was updated.DeclarationSwift public let cue: TextTrackCue
