Enable -Wall, -Wextra and -Wunused-variable for talk/ on clang.

Also removes one case of unused-variable.

BUG=3220
R=henrike@webrtc.org, kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15619005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6297 4adac7df-926f-26a2-2b94-8c16560cd09d