Clean up RTCPSender.

Reformat to current code style, remove non-const references, use
scoped_ptr, remove empty comments and dead code, etc..

BUG=
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49019004

Cr-Commit-Position: refs/heads/master@{#9086}

webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h

@@ -342,9 +342,6 @@ class ModuleRtpRtcpImpl : public RtpRtcp {
   // Get remote SequenceNumber.
   uint16_t RemoteSequenceNumber() const;
 
-  // Only for internal testing.
-  uint32_t LastSendReport(int64_t& last_rtcptime);
-
   RTPSender rtp_sender_;
 
   RTCPSender rtcp_sender_;