Add pixel test for <select> -webkit-autofil style

R=tkent@chromium.org
BUG=58719

Review URL: https://codereview.chromium.org/216923003

git-svn-id: svn://svn.chromium.org/blink/trunk@170623 bbb929c8-8fbe-4397-9dbb-9b2b20218538