Fix for RTX in combination with pacing.

Retransmissions didn't get sent over RTX when pacing was enabled since
the pacer didn't keep track of whether a packet was a retransmit or not.

BUG=1811
TEST=trybots
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/3779004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5117 4adac7df-926f-26a2-2b94-8c16560cd09d
13 files changed