Make "descriptors" argument of FontFace constructor optional

This reflects recent spec update.

spec: http://dev.w3.org/csswg/css-font-loading/#fontface-interface

BUG=
TEST=fast/css/font*

Review URL: https://codereview.chromium.org/335333002

git-svn-id: svn://svn.chromium.org/blink/trunk@176329 bbb929c8-8fbe-4397-9dbb-9b2b20218538