Creating a test helper class TimestampJumpRtpGenerator

This class provides a way to test with an RTP sequence that make an
arbitrary jump in the timestamp series.

R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23679004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7236 4adac7df-926f-26a2-2b94-8c16560cd09d