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}
diff --git a/webrtc/api/rtpreceiver.h b/webrtc/api/rtpreceiver.h
index b581857..2e7339d 100644
--- a/webrtc/api/rtpreceiver.h
+++ b/webrtc/api/rtpreceiver.h
@@ -56,6 +56,9 @@
 
   void Stop() override;
 
+  RtpParameters GetParameters() const override;
+  bool SetParameters(const RtpParameters& parameters) override;
+
  private:
   void Reconfigure();
 
@@ -89,6 +92,9 @@
 
   void Stop() override;
 
+  RtpParameters GetParameters() const override;
+  bool SetParameters(const RtpParameters& parameters) override;
+
  private:
   std::string id_;
   uint32_t ssrc_;