Element.setScrollOffset should integrate with RACUN

The setScrollOffset code was either generating a synchronous
invalidation or using repaint-after-compositing-update. This
change removes the former code path.

While the overall test invalidation area is the same in all
tests, we generate more invalidations. The extra invalidations
are caused by forcing a subtree walk of the shifted scrollable
areas and thus forcing an invalidation for some of the descendants.

Review URL: https://codereview.chromium.org/541273002

git-svn-id: svn://svn.chromium.org/blink/trunk@181485 bbb929c8-8fbe-4397-9dbb-9b2b20218538