Add thread/sequence checks to ModuleRtpRtcpImpl.

This ended up with needing to fork the current implementation
in order to not break downstream projects that were inheriting
from it. While those get updated, we'll move on with the forked
class.

Bug: webrtc:11581,b/8278269
Change-Id: I05b596cbda71aa5b72894c31a7119d17d4761883
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175500
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31334}

modules/rtp_rtcp/source/rtcp_sender.h

@@ -38,10 +38,10 @@
 
 namespace webrtc {
 
-class ModuleRtpRtcpImpl;
+class RTCPReceiver;
 class RtcEventLog;
 
-class RTCPSender {
+class RTCPSender final {
  public:
   struct FeedbackState {
     FeedbackState();
@@ -61,7 +61,7 @@ class RTCPSender {
     std::vector<rtcp::ReceiveTimeInfo> last_xr_rtis;
 
     // Used when generating TMMBR.
-    ModuleRtpRtcpImpl* module;
+    RTCPReceiver* receiver;
   };
 
   explicit RTCPSender(const RtpRtcp::Configuration& config);