Improve persistence and WebFont compatibility of font patch

Sid Stamm reviewed my font count limiting patch and suggested that the nsPrefShell is the wrong place to store font counts, since it may be cleared in circumstances such as refreshes/redirects. We probably want something that has a similar lifetime to cookies.. Perhaps the Content Prefs database?

Additionally, we should be passing a bit down to prevent the font limits from applying to remotely-sourced WebFonts (from @font-face).