Remove unused CSSFontSelector argument from styleForDocument

This argument isn't used anymore. I also removed a comment that didn't
make sense (it was from seamless).

Review URL: https://codereview.chromium.org/238353004

git-svn-id: svn://svn.chromium.org/blink/trunk@171684 bbb929c8-8fbe-4397-9dbb-9b2b20218538