MediaKeySession：closed 属性
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2019年3月.
MediaKeySession 接口的 closed 只读属性返回用于指示 MediaKeySession 何时关闭的 Promise。该 promise 只会被兑现且永远不会被拒绝。关闭会话意味着与其关联的许可证和密钥不再有效，并且将无法用于解密媒体数据。