Add testing for overlay scrollbars.

The code for painting the scrollbars and computing their
size isn't used when compositing, but we still get some
testing out of the track size and computing LTR/RTL.

The RTL detection is currently broken as seen by the
RTL scrollbar showing up on the left edge.  I'll fix that
in a follow up.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/23969003

git-svn-id: svn://svn.chromium.org/blink/trunk@157262 bbb929c8-8fbe-4397-9dbb-9b2b20218538