Add a failing test to demonstrate missing overlay scrollbars.

Reflected overlay scrollbars don't always appear if the scrollable area
is not composited. This CL adds a test that demonstrates this.

BUG=396775

Review URL: https://codereview.chromium.org/411973003

git-svn-id: svn://svn.chromium.org/blink/trunk@178836 bbb929c8-8fbe-4397-9dbb-9b2b20218538