The HTML spec allows documents to be discarded to reclaim resources (https://html.spec.whatwg.org/multipage/#discard-a-document). In particular, documents that aren't active may be discarded (https://html.spec.whatwg.org/multipage/browsers.html#the-session-history-of-browsing-contexts:discard-a-document).

#11893 contains a partial implementation.

cc @Ms2ger @jdm @ConnorGBrewster @metajack