Support size<4 rendering of listbox <select>.

AFAIK, this CL will break scrollbar rendering on SnowLeopard.  It should
be a minor problem according to the OS market share.

BUG=4579

Review URL: https://codereview.chromium.org/580013002

git-svn-id: svn://svn.chromium.org/blink/trunk@182276 bbb929c8-8fbe-4397-9dbb-9b2b20218538