Eliminate FontDescription::setStyle(bool).

It's much clearer to pass a FontStyle enum at the call site, and only a
handful of callers don't do this already.

BUG=

Review URL: https://codereview.chromium.org/213683008

git-svn-id: svn://svn.chromium.org/blink/trunk@170722 bbb929c8-8fbe-4397-9dbb-9b2b20218538