Signal when shutting down DirectTransport.

Avoids starting the network thread when there are no packets to be read.
This allows the transport to shut down directly, which makes tests using
it able to quit faster, and not have to wait up to 10ms.

BUG=
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2010004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@4524 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed