Move default style for <dialog> to html.css

This is a follow-up of Blink r181385.

BUG=402536

Review URL: https://codereview.chromium.org/553353002

git-svn-id: svn://svn.chromium.org/blink/trunk@181710 bbb929c8-8fbe-4397-9dbb-9b2b20218538