Fetch keepalive duration on shutdown

Controls the duration (in seconds) that keepalive requests are allowed to prevent the browser from completing its shutdown. If you configure this policy, the browser blocks completing shutdown while it processes any outstanding keepalive requests (see https://fetch.spec.whatwg.org/#request-keepalive-flag) up to the maximum period of time specified by this policy. If you disable or don't configure this policy, the default value of 0 seconds is used, and the outstanding keepalive requests are immediately cancelled during browser shutdown.