Disable TestDrain test on memcheck bots.

P2PTransportChannelMultihomedTest.TestDrain is flaky on memcheck bots,
likely the test timeout is insufficient for memcheck which incurs a
serious slowdown.

BUG=2409,3447
R=minyue@webrtc.org
TBR=jiayl@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17339004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7081 4adac7df-926f-26a2-2b94-8c16560cd09d