Break out RtpClock to system_wrappers and make it more generic.

The goal with this new clock interface is to have something which is used all
over WebRTC to make it easier to switch clock implementation depending on where
the components are used. This is a first step in that direction.

Next steps will be to, step by step, move all modules, video engine and voice
engine over to the new interface, effectively deprecating the old clock
interfaces. Long-term my vision is that we should be able to deprecate the clock
of WebRTC and rely on the user providing the implementation.

TEST=vie_auto_test, rtp_rtcp_unittests, trybots

Review URL: https://webrtc-codereview.appspot.com/1041004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3381 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h

@@ -484,7 +484,7 @@ class ModuleRtpRtcpImpl : public RtpRtcp {
   RTCPReceiver              rtcp_receiver_;
 
   bool                      owns_clock_;
-  RtpRtcpClock&             clock_;
+  Clock&                    clock_;
 
  private:
   int64_t RtcpReportInterval();