Remove unused members in CSSFontFaceSource.h

Just something I noticed while reading the code.

R=eae
BUG=

Review URL: https://codereview.chromium.org/69953014

git-svn-id: svn://svn.chromium.org/blink/trunk@161947 bbb929c8-8fbe-4397-9dbb-9b2b20218538