(Based on discussion with rdsmith and mmenke)

This would require (not necessarily in order):
(1) Moving the DRP logic from
 DRPNetworkDelegate::OnBeforeSendHeaders*() to DRPProxyDelegate.
(2) Moving OnBeforeSendHeaders*() from NetworkDelegate to net::ProxyDelegate.
(3) Moving the NotifyBeforeSendHeadersCallback() from UrlRequestHttpJob
 to HttpNetworkTransaction.

This refactoring would reduce NetworkDelegate upcall by one.