Cast payload types to int for logging.

uint8_t gets interpreted as char and printed as such, instead of being
printed in decimal, casting them to int allows us to read what payload
types are actually used without converting them from ASCII first.

BUG=chromium:390874
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/13919004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6662 4adac7df-926f-26a2-2b94-8c16560cd09d