Main frame scroll snap tests have become flaky after http://trac.webkit.org/projects/webkit/changeset/188793
Steps to reproduce:
1. Edit tiled-drawing/scrolling/scroll-snap/scroll-snap-mandatory-mainframe-vertical.html (this makes it 100% reproducible for me).
2. Add "document.body.offsetTopl" at the beginning on onLoad function.
3. Run the test.
What happens is that callAfterSCrollingCompletes callback is called before scrolling completes.
Alternative steps to reproduce:
run-webkit-tests -f --repeat 50 tiled-drawing/scrolling/scroll-snap
Marked as flaky in r188896