Enable resetable and String prefs.

This allows both boolean and string-type preferences. It
also implements a system where prefs that are read from a
configuration file can be reset back to their initial value,
which is useful in a number of cases e.g. when running tests
to ensure that each test starts with the same values for
the prefs.

components/script/dom/htmliframeelement.rs

@@ -40,7 +40,7 @@ use util::str::DOMString;
 use util::str::{self, LengthOrPercentageOrAuto};
 
 pub fn mozbrowser_enabled() -> bool {
-    prefs::get_pref("dom.mozbrowser.enabled").unwrap_or(false)
+    prefs::get_pref("dom.mozbrowser.enabled").as_boolean().unwrap_or(false)
 }
 
 #[derive(HeapSizeOf)]