Add a DOMUI datasource that will handle the net-internals page.
Right now it is bound to the temporary URL "chrome://net2", since the frontend is under construction.
(Loading this page right now just shows a dummy HTML page.)
Note that the network stuff needs to run on the IO thread, however the DOMUI infrastructure is all set up to run on the UI thread.
I work around this by proxying things back and forth through the IO thread. Hopefully in subsequent iterations this overhead can be removed, by exposing a sort of "IO thread DOMUI".