Disable P2PTransportChannelMultihomedTest.TestFailover under Memcheck

BUG=3447
R=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15689004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6327 4adac7df-926f-26a2-2b94-8c16560cd09d