If no frame has been decoded the jitter buffer might generate huge
erroneous NACK lists.
Adds a couple of new jitter buffer unittests (some ported from
jitter_buffer_test.cc).
Adds a test to the VCM robustness tests.
BUG=226
TEST=VCMRobustnessTest, TestJitterBufferFull, TestNackListFull, TestNackBeforeDecode, TestNormalOperation
Review URL: http://webrtc-codereview.appspot.com/352002
git-svn-id: http://webrtc.googlecode.com/svn/trunk@1412 4adac7df-926f-26a2-2b94-8c16560cd09d