Remove above/below scroll areas (dead code)

There is a single layer for overscroll areas (not one above and one
below).

BUG=133097

Review URL: https://chromiumcodereview.appspot.com/22300002

git-svn-id: svn://svn.chromium.org/blink/trunk@155572 bbb929c8-8fbe-4397-9dbb-9b2b20218538