Fire onscroll events at requestAnimationFrame timing

This is the best time to fire the event since that's when the scroll state
is synced back from the compositor for threaded scrolling and it allows the
page to take an action at the right spot to avoid flickering if doing
something like infinite scroll.

BUG=299317

Review URL: https://codereview.chromium.org/25372003

git-svn-id: svn://svn.chromium.org/blink/trunk@159291 bbb929c8-8fbe-4397-9dbb-9b2b20218538