The argument to `local()` is not a font family, but the name of a
»single font face within a larger family« according to
http://www.w3.org/TR/css3-fonts/#src-desc.
The previous implementation of `FontCache::run` would panic if the argument to
`local()` would not be the name of a font family present in `self.web_families`.
That happened since `FontCacheTask` tried to use the argument to `local()` as a
key for `self.web_families`, although it previously correctly initialized a
value for the `family` field of the `AddWebFont` command.