Composited scrolling should not affect normal-flow status

With the old composited scrolling path, we would promote 
the overflow scrolling div to a stacking container.
Before doing this, we very carefully checked that we
would not break stacking by doing so.

With "universal" accelerated overflow scrolling, we do
not create a stacking conatiner, so we naturally don't
check if promoting will change stacking. In this new
world, composited scrolling has no bearing on stacking,
so it doesn't makes sense to factor it into the
normal-flow-only logic.

R=hartmanng@chromium.org
BUG=365790

Review URL: https://codereview.chromium.org/247963003

git-svn-id: svn://svn.chromium.org/blink/trunk@172293 bbb929c8-8fbe-4397-9dbb-9b2b20218538