Mark all virtual overrides in the hierarchies of UdpTransportData and
UdpSocketWrapper 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.

This also removes an unused function.

BUG=none
TEST=none
R=henrik.lundin@webrtc.org, henrikg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23579004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7195 4adac7df-926f-26a2-2b94-8c16560cd09d