Exclude flaky libjingle_peerconnection_unittest test for Memcheck.

The PeerConnectionEndToEndTest.DataChannelIdAssignment test fails
flakily like this:
[----------] 1 test from PeerConnectionEndToEndTest
[ RUN      ] PeerConnectionEndToEndTest.DataChannelIdAssignment
WARNING: no real random source present!
../../talk/app/webrtc/test/peerconnectiontestwrapper.cc:216: Failure
Value of: CheckForConnection()
  Actual: false
Expected: true
[  FAILED  ] PeerConnectionEndToEndTest.DataChannelIdAssignment (13215 ms)
[----------] 1 test from PeerConnectionEndToEndTest (13223 ms total)

TBR=wu@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/20759004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6518 4adac7df-926f-26a2-2b94-8c16560cd09d