Move VirtualSocket into the .h file to allow unit tests more control over behavior.

BUG=3927
R=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/31289004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7935 4adac7df-926f-26a2-2b94-8c16560cd09d