Mac: Update -webkit-appearance:menulist-button implementation to match to Lion+.

- Don't draw fixed color border, use CSS border with #a6a6a6.
- Don't draw gradient background.
  The gradation direction was opposite to the native menulist button.
  The native menulist button doesn't have shadow on left/right sides.
  We use flat #f8f8f8 background color, not CSS gradient because
  <select style="background-color:red"> would not work if we specified
  CSS gradient in UA stylesheet.
- Don't draw hollowed vertical line.

BUG=345609
TEST=automated

Review URL: https://codereview.chromium.org/174163010

git-svn-id: svn://svn.chromium.org/blink/trunk@168130 bbb929c8-8fbe-4397-9dbb-9b2b20218538