Page Lifecycle

https://developers.google.com/web/updates/2018/07/page-lifecycle-api

https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API#Policies_in_place_to_aid_background_page_performance

添加了 onfreeze and onresume and wasDiscarded(被丢弃后恢复的页面？) attributes

不同的状态下应该执行不同的动作。