changed
Description:
Emitted when hsts_enforcer changes.
 If a policy has been added, new_policy will contain the newly-added policy and old_policy will be null.
      If a policy has been deleted, old_policy will contain the to-be-deleted policy and new_policy will be null
      . If a policy has been changed, old_policy will contain its old value, and new_policy its new value.
Note that you shouldn't modify the policies from a callback to this signal.
Parameters:
| old_policy | the old HSTSPolicy value | 
| new_policy | the new HSTSPolicy value |