Mark virtual overrides of ViENetwork and VoENetwork as such.

This will make further changes to these classes safer by ensuring that the
compile breaks if the base class changes and not all overrides are fixed.

BUG=none
TEST=none
R=henrikg@webrtc.org, mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/26439004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7305 4adac7df-926f-26a2-2b94-8c16560cd09d