BackgroundFetchRegistration：uploaded 属性
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
备注： 此特性在 Web Worker 中可用。
BackgroundFetchRecord 接口的只读属性 uploaded 返回成功发送的字节数。该属性的初始值为 0。
如果此属性的值发生变化，将在关联的 BackgroundFetchRegistration 对象上触发 progress 事件。