Cascade top document's level to subframes with no explicitly assigned level.
    
    Keep in mind that if a subframe domain has a level assigned that level
    is set and not the one of the top level window Origin.
    
    This is a workaround for Github #133
    
    Known bug:
    
    Keep in mind that the code treats some nested iframes incorrectly.
    Suppose that there is an iframe from domain C nested in an iframe from
    domain B that is iself nested in a visited domain A.
    
    +------------------------------------------------------------+
    | visited domain a.example                                   |
    |                                                            |
    | +--------------------------------------------------------+ |
    | | iframe from domain b.example                           | |
    | |                                                        | |
    | | +----------------------------------------------------+ | |
    | | | iframe from domain c.example                       | | |
    | | |                                                    | | |
    | | +----------------------------------------------------+ | |
    | |                                                        | |
    | +--------------------------------------------------------+ |
    +------------------------------------------------------------+
    
    Suppose that B has a user-defined specific level settings, and C does
    not have a user-defined specific level settings. The iframe of domain B
    gets the user-defined settings for domain B but the iframe from domain C
    is set with the level of domain A.