Navigation Error Logging spec update

I added two methods to the interface to allow reporting of errors in real
time to a report url as per *ACTION-117 -* *Add method to allow ability to
send to a third party url.*

https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/NavigationErrorLogging/Overview.html

Please review and let me know if you have any concerns.

Arvind