Fix for RenderListBox not showing vertical scrollbar with overlay-scrollbar enabled.

The scrollbars does not paint as verticalScrollbarWidth() returns 0 in paint in case of overlay-scrollbars.
We should use actual scrollbar width while placing and painting the scrollbar.

BUG=385463

Review URL: https://codereview.chromium.org/337323002

git-svn-id: svn://svn.chromium.org/blink/trunk@176395 bbb929c8-8fbe-4397-9dbb-9b2b20218538