Fix double backslashes in incoming_video_stream.cc

Originally uploaded in https://codereview.appspot.com/149160043/.

TBR=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/27859004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7541 4adac7df-926f-26a2-2b94-8c16560cd09d