必须使用 PerformanceResourceTiming 对象作为接收者调用此属性 getter。
PerformanceResourceTiming
新增于: v18.2.0, v16.17.0
表示获取资源大小（以八位字节为单位）的数值。大小包括响应头字段加上响应负载正文。
¥A number representing the size (in octets) of the fetched resource. The size includes the response header fields plus the response payload body.