Currently, servo panics for me when loading something like this:
```
@font-face {
font-family: "test family";
src: local(test font face);
}
```
That's due to a bug in `FontCacheTask`. `FontCacheTask` tries to get the value for the key
"test font face" from `self.web_families`, but previously initialized a value for the key "test family".
These two commits add an awkward test and fix the bug by not shadowing the variable `family_name`. Since the argument to `local()` should explicitly not be the name of a font family, the previous variable name was wrong and misleading anyways.
Source-Repo: https://github.com/servo/servo
Source-Revision: 8e75a05e6bea78d388e6409020f3279a090c9168
7a094fe30e