Refactoring PayloadRouter.

- Move PayloadRouter to RtpTransportControllerInterface.
- Move RetransmissionLimiter inside RtpTransportControllerSend from
  VideoSendStreamImpl.
- Move video RTP specifics into PayloadRouter, in particular ownership
  of the RTP modules.
- PayloadRouter now contains all video specific RTP code, and will be
  renamed in a follow-up to VideoRtpSender.
- Introduce VideoRtpSenderInterface.

Bug: webrtc:9517
Change-Id: I1c7b293fa6f9c320286c80533b3c584498034a38
Reviewed-on: https://webrtc-review.googlesource.com/88240
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24009}

modules/video_coding/include/video_coding.h

@@ -46,9 +46,9 @@ class EventFactory {
 
 class EventFactoryImpl : public EventFactory {
  public:
-  virtual ~EventFactoryImpl() {}
+  ~EventFactoryImpl() override {}
 
-  virtual EventWrapper* CreateEvent() { return EventWrapper::Create(); }
+  EventWrapper* CreateEvent() override;
 };
 
 // Used to indicate which decode with errors mode should be used.