Remove FontDescription::isSpecifiedFont.

We actually never use it in a meaningful way.

R=eae,eseidel
BUG=

Review URL: https://codereview.chromium.org/195893047

git-svn-id: svn://svn.chromium.org/blink/trunk@169586 bbb929c8-8fbe-4397-9dbb-9b2b20218538