新增于: v16.17.0
即将发送 fetch 请求之前的高解析度毫秒时间戳。
如果资源没有被工作进程截获，该属性将始终返回 0。
Added in: v16.17.0
The high resolution millisecond timestamp at immediately before dispatching
the fetch request. If the resource is not intercepted by a worker the property
will always return 0.