Add CSSFontSelector::removeFontFaceRule()

This allows StyleResolver to avoid resetting CSSFontSelector when styles are removed.

BUG=305885
TEST=none, the new method is not used yet

Review URL: https://codereview.chromium.org/68933007

git-svn-id: svn://svn.chromium.org/blink/trunk@162221 bbb929c8-8fbe-4397-9dbb-9b2b20218538