Calculating round-trip-time in send-only channel in VoE.

TESTS=built chromium and tested with 1:1 hangout call

BUG=
R=stefan@webrtc.org, xians@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23489004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7147 4adac7df-926f-26a2-2b94-8c16560cd09d