This is a related to bug 652889 , tracking changes to RecentTabHelper.
When user pressed Download button for the page, the tab helper will start observing the process of loading the page. If the good moment to snapshot a page will arise, it'll do so and tell RequestCoordinator. If such opportunity does not arise (tab renavigates or is closed) - it tells RequestCoordinator to work on that request. If Chrome is evicted from RAM, the RequestCoordinator picks up the request later.
Comment 1 by bugdroid1@chromium.org
, Oct 13 2016