Adding getParameters/setParameters APIs to RtpReceiver.

This is similar to how a "receive" method is used to apply
RtpParameters to an RtpReceiver in ORTC. Currently, SetParameters
doesn't allow changing the parameters, so the main use of the API is
to retrieve the set of configured codecs. But other uses will likely
be made possible in the future.

R=glaznev@webrtc.org, pthatcher@webrtc.org, tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1917193008 .

Cr-Commit-Position: refs/heads/master@{#12761}

webrtc/api/rtpreceiverinterface.h

@@ -33,6 +33,12 @@ class RtpReceiverInterface : public rtc::RefCountInterface {
 
   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*.
+  virtual RtpParameters GetParameters() const = 0;
+  virtual bool SetParameters(const RtpParameters& parameters) = 0;
+
  protected:
   virtual ~RtpReceiverInterface() {}
 };
@@ -42,6 +48,8 @@ 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()
 
 }  // namespace webrtc