Separating internal and external methods of RtpSender/RtpReceiver.

This moves the implementation specific methods to separate classes
(RtpSenderInternal/RtpReceiverInternal) so that the interface classes
represent the interface that external applications can rely on.

The reason this wasn't done earlier was that PeerConnection needed
to store proxy pointers, but also needed to access implementation-
specific methods on the underlying objects. This is now possible
by using "RtpSenderProxyWithInternal<RtpSenderInternal>", which is a proxy
that implements RtpSenderInterface but also provides direct access
to an RtpSenderInternal.

Review-Url: https://codereview.webrtc.org/2023373002
Cr-Commit-Position: refs/heads/master@{#13056}

webrtc/api/rtpreceiverinterface.h

@@ -31,8 +31,6 @@ class RtpReceiverInterface : public rtc::RefCountInterface {
   // to uniquely identify a receiver until we implement Unified Plan SDP.
   virtual std::string id() const = 0;
 
-  virtual void Stop() = 0;
-
   // The WebRTC specification only defines RTCRtpParameters in terms of senders,
   // but this API also applies them to receivers, similar to ORTC:
   // http://ortc.org/wp-content/uploads/2016/03/ortc.html#rtcrtpparameters*.
@@ -47,7 +45,6 @@ class RtpReceiverInterface : public rtc::RefCountInterface {
 BEGIN_SIGNALING_PROXY_MAP(RtpReceiver)
 PROXY_CONSTMETHOD0(rtc::scoped_refptr<MediaStreamTrackInterface>, track)
 PROXY_CONSTMETHOD0(std::string, id)
-PROXY_METHOD0(void, Stop)
 PROXY_CONSTMETHOD0(RtpParameters, GetParameters);
 PROXY_METHOD1(bool, SetParameters, const RtpParameters&)
 END_SIGNALING_PROXY()