Called when a content pref is set to a different value.
| aGroup | the group to which the pref belongs, or null if it's a global pref (applies to all sites) |
| aName | the name of the pref that was set |
| aValue | the new value of the pref |
Called when a content pref is removed.
| aGroup | the group to which the pref belongs, or null if it's a global pref (applies to all sites) |
| aName | the name of the pref that was removed |