Add test NetEqDecodingTest.CngFirst

This CL adds a test to verify that it is ok to start the stream with
a comfort noise packet.

BUG=4021
R=minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/29079004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7769 4adac7df-926f-26a2-2b94-8c16560cd09d