Propagating RTT from send-only channel to receive-only channel.

This is important for obtaining ntp time at receiver-only channel, which does not have RTT directly.

BUG=3978

TEST=chromium with hangout calls
R=henrika@webrtc.org, mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/29989004

Cr-Commit-Position: refs/heads/master@{#9186}
11 files changed