Refactor some receive-side stats.

Removes polling of CName as well as receive codec statistics in favor of
internal callbacks keeping a statistics struct up to date.

R=mflodman@webrtc.org, stefan@webrtc.org
BUG=1667

Review URL: https://webrtc-codereview.appspot.com/28259005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7950 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/rtp_rtcp/source/receive_statistics_unittest.cc

@@ -169,6 +169,8 @@ TEST_F(ReceiveStatisticsTest, RtcpCallbacks) {
       ++num_calls_;
     }
 
+    virtual void CNameChanged(const char* cname, uint32_t ssrc) OVERRIDE {}
+
     uint32_t num_calls_;
     uint32_t ssrc_;
     RtcpStatistics stats_;