ACTION-177: Add an API to let a site request a web-wide exception

6.7 Web-wide exceptions

Users may wish to configure exceptions for a certain trusted tracker across several or all sites.

User agents may instantiate NavigatorDoNotTrack.requestWebWideTrackingException. Sites should test for the existence of requestWebWideTrackingException before calling the method. If an exception is granted in this context and the user-agent stores that preference, a user agent may send a DNT:0 header even if a tracking preference isn't expressed for other requests.

A web-wide exception request can be initiated even when navigator.doNotTrack is null.

[NoInterfaceObject]
interface NavigatorDoNotTrack {
    void requestWebWideTrackingException (DomainString, TrackingResponseCallback callback, optional  explanationString, optional  detailURI);
};

   - DomainString, a single domain signifying the origin of the domain receiving the web-wide exception.
   - Note - all like named fields carry the same definitions as set forth in requestSiteSpecificTrackingException.

A separate call must be made for each DomainString and the requester origin should match the DomainString.

Multiple calls can be facilitated through nested iFrames for each domain, similar to the NAI opt-out page individual domain call routines to set opt-out cookies for each domain.