Add support for bi-directional simulations by having both an uplink and a downlink.

Partially make PacketReceiver a source which should be possible to hook up to the network.

Require that flow ids are set in all constructors.

BUG=4173
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/36059004

Cr-Commit-Position: refs/heads/master@{#8367}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8367 4adac7df-926f-26a2-2b94-8c16560cd09d
7 files changed