Revert of Use VoiceChannel/VideoChannel directly from RtpSender/RtpReceiver. (patchset #3 id:40001 of https://codereview.webrtc.org/2046173002/ )

Reason for revert:
Broke peerconnection_unittest somehow, due to introduction of thread check. Will fix and reland.

Original issue's description:
> Use VoiceChannel/VideoChannel directly from RtpSender/RtpReceiver.
>
> This eliminates the need for the extra layer of indirection provided by
> mediastreamprovider.h. It will thus make it easier to implement new
> functionality in RtpSender/RtpReceiver.
>
> It also brings us one step closer to the end goal of combining "senders"
> and "send streams". Currently the sender still needs to go through the
> BaseChannel and MediaChannel, using an SSRC as a key.
>
> R=pthatcher@webrtc.org
>
> Committed: https://crrev.com/bc5831999d3354509d75357b659b4bb8e39f8a6c
> Cr-Commit-Position: refs/heads/master@{#13285}

TBR=pthatcher@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review-Url: https://codereview.webrtc.org/2099843003
Cr-Commit-Position: refs/heads/master@{#13286}
diff --git a/webrtc/api/rtpreceiverinterface.h b/webrtc/api/rtpreceiverinterface.h
index 53a3739..4943023 100644
--- a/webrtc/api/rtpreceiverinterface.h
+++ b/webrtc/api/rtpreceiverinterface.h
@@ -18,7 +18,6 @@
 
 #include "webrtc/api/mediastreaminterface.h"
 #include "webrtc/api/proxy.h"
-#include "webrtc/api/rtpparameters.h"
 #include "webrtc/base/refcount.h"
 #include "webrtc/base/scoped_ref_ptr.h"
 #include "webrtc/pc/mediasession.h"
@@ -27,12 +26,6 @@
 
 class RtpReceiverObserverInterface {
  public:
-  // Note: Currently if there are multiple RtpReceivers of the same media type,
-  // they will all call OnFirstPacketReceived at once.
-  //
-  // In the future, it's likely that an RtpReceiver will only call
-  // OnFirstPacketReceived when a packet is received specifically for its
-  // SSRC/mid.
   virtual void OnFirstPacketReceived(cricket::MediaType media_type) = 0;
 
  protected:
@@ -43,9 +36,6 @@
  public:
   virtual rtc::scoped_refptr<MediaStreamTrackInterface> track() const = 0;
 
-  // Audio or video receiver?
-  virtual cricket::MediaType media_type() const = 0;
-
   // Not to be confused with "mid", this is a field we can temporarily use
   // to uniquely identify a receiver until we implement Unified Plan SDP.
   virtual std::string id() const = 0;
@@ -56,10 +46,10 @@
   virtual RtpParameters GetParameters() const = 0;
   virtual bool SetParameters(const RtpParameters& parameters) = 0;
 
-  // Does not take ownership of observer.
-  // Must call SetObserver(nullptr) before the observer is destroyed.
   virtual void SetObserver(RtpReceiverObserverInterface* observer) = 0;
 
+  virtual cricket::MediaType media_type() = 0;
+
  protected:
   virtual ~RtpReceiverInterface() {}
 };
@@ -67,11 +57,11 @@
 // Define proxy for RtpReceiverInterface.
 BEGIN_SIGNALING_PROXY_MAP(RtpReceiver)
 PROXY_CONSTMETHOD0(rtc::scoped_refptr<MediaStreamTrackInterface>, track)
-PROXY_CONSTMETHOD0(cricket::MediaType, media_type)
 PROXY_CONSTMETHOD0(std::string, id)
 PROXY_CONSTMETHOD0(RtpParameters, GetParameters);
 PROXY_METHOD1(bool, SetParameters, const RtpParameters&)
 PROXY_METHOD1(void, SetObserver, RtpReceiverObserverInterface*);
+PROXY_METHOD0(cricket::MediaType, media_type);
 END_SIGNALING_PROXY()
 
 }  // namespace webrtc