Add clockdrift to RtpGenerator

RtpGenerator is a help class for NetEq testing. This change
add the possibility to simulate clockdrift. If no clockdrift is
set, the default is 0 (i.e., no drift).

R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2175005

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@4680 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed