Set NACKed packet to -1 in TestNackRetransmission.
Set NACKed packet to -1 in TestNackRetransmission.

Zero is a valid sequence number which may occur even if there are no
retransmissions, this caused the test to flake as an incoming packet
would be mistaken for a retransmission.

BUG=2830
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/7509005

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e7223e7795b8581575fcaf58be971f2ed8f8af2e

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc