Remove failed CSSFontFaceSource from CSSFontFace

If a CSSFontFaceSource failed to generate valid font data, there is no
reason to keep it. This also makes isValid() constant-time.

BUG=
TEST=covered by existing tests

Review URL: https://codereview.chromium.org/141513010

git-svn-id: svn://svn.chromium.org/blink/trunk@166283 bbb929c8-8fbe-4397-9dbb-9b2b20218538