Scrollbar: Paint the forward arrow button with disabled appearance even if the current position is larger than the maximum position.

We painted the disabled arrow button only if the current position was
equal to the maximum position. RenderListBox (<select multiple>)
provides a negative maxmimum position when its options are fewer than
its height.

This CL has no test. Unfortunately we don't paint disabled arrow
buttons in layout test mode.

BUG=338994

Review URL: https://codereview.chromium.org/148273011

git-svn-id: svn://svn.chromium.org/blink/trunk@166103 bbb929c8-8fbe-4397-9dbb-9b2b20218538