WebRTCDemo: correct set trace filter operation.

BUG=3285
TEST=Manul Test
R=fischman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17389004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6051 4adac7df-926f-26a2-2b94-8c16560cd09d