Tony also noted that it is unlikely that browsers can directly export AFT. This is because browsers paint proactively. That is, browsers paint not when they are certain there is content to be painted, but rather when there is a fair chance there is content to be painted. The rationale of course, is to paint content as early as possible.
webpagetest.org (WPT) currently hooks Windows GDI calls, so it only captures frames when there is an OS paint call. This should make computing AFT based on screen shots collected in response to browser paint calls an easy transition.