Mark template class RefCountedObject as final.

pc/rtpreceiver.h

Index: pc/rtpreceiver.h
diff --git a/pc/rtpreceiver.h b/pc/rtpreceiver.h
index 6b9ca99bac0b3383aee89409353c0626a816ef61..7a1d836c45acc0258f20fadc5cc849e44665835a 100644
--- a/pc/rtpreceiver.h
+++ b/pc/rtpreceiver.h
@@ -41,7 +41,7 @@ class RtpReceiverInternal : public RtpReceiverInterface {
 
 class AudioRtpReceiver : public ObserverInterface,
                          public AudioSourceInterface::AudioObserver,
-                         public rtc::RefCountedObject<RtpReceiverInternal>,
+                         public RtpReceiverInternal,
                          public sigslot::has_slots<> {
  public:
   // An SSRC of 0 will create a receiver that will match the first SSRC it
@@ -105,7 +105,7 @@ class AudioRtpReceiver : public ObserverInterface,
   bool received_first_packet_ = false;
 };
 
-class VideoRtpReceiver : public rtc::RefCountedObject<RtpReceiverInternal>,
+class VideoRtpReceiver : public RtpReceiverInternal,
                          public sigslot::has_slots<> {
  public:
   // An SSRC of 0 will create a receiver that will match the first SSRC it