This policy allows an admin to specify that a page may show popups during its unloading.
When the policy is set to enabled, pages are allowed to show popups while they are being unloaded.
When the policy is set to disabled or not set, pages are not allowed to show popups while they are being unloaded, as per the spec (https://html.spec.whatwg.org/#apis-for-creating-and-navigating-browsing-contexts-by-name).
This policy will be removed in Chrome 88.
See https://www.chromestatus.com/feature/5989473649164288 .