This property getter must be called with the PerformanceResourceTiming object as the receiver.
PerformanceResourceTiming
新增于: v18.2.0, v16.17.0
表示获取的资源大小（以字节为单位）的数字。大小包括响应头字段以及响应负载主体。
【A number representing the size (in octets) of the fetched resource. The size includes the response header fields plus the response payload body.】