Enable and isolate Firefox connection prediction

In #16625 (moved), we found that Firefox's network prediction was partially disabled due to our use of SOCKS proxy settings, and also that it wasn't properly getting isolated in the cache or elsewhere.

We should try to enable this, though, as it will likely represent a significant decrease in page load times. If we enable isolation, the only real privacy issue is prediction for link hover, and that can be easily disabled in Predictor::PredictForLink() (there already is a check to disable it for HTTPS links).