Introduce a DefaultAddressProvider such that rtc::Network can't access other part of NetworkManager. This also removes the hack of generating the loopback address. The dependency has been removed by https://codereview.chromium.org/1417023003/ BUG=webrtc:5061 R=pthatcher@webrtc.org Review URL: https://codereview.webrtc.org/1411253008 . Cr-Commit-Position: refs/heads/master@{#10590}