Make RenderObject::compositorDrivenAcceleratedScrollingEnabled smarter

This function turned up in one of our profiles. I'm not sure this change makes
much of a difference, but we might as well not have silly code on the profile.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/187793005

git-svn-id: svn://svn.chromium.org/blink/trunk@168596 bbb929c8-8fbe-4397-9dbb-9b2b20218538