Regression: <optgroup> doesn't let <option> inherit style from <select>

options inside optgroups weren't inheriting styles when displayed in popup menus because optgroup wasn't creating a renderStyle.

BUG=395613

Review URL: https://codereview.chromium.org/402393002

git-svn-id: svn://svn.chromium.org/blink/trunk@178886 bbb929c8-8fbe-4397-9dbb-9b2b20218538