Enable setting the maximum bitrate limit in RtpSender.

This change allows the application to limit the bitrate of the outgoing
audio and video streams at runtime. The API roughly follows the WebRTC
API draft, defining the RTCRtpParameters structure witn exactly one
encoding (simulcast streams are not exposed in the API for now).
(https://www.w3.org/TR/webrtc/#idl-def-RTCRtpParameters)

BUG=

Review URL: https://codereview.webrtc.org/1788583004

Cr-Commit-Position: refs/heads/master@{#12025}
diff --git a/webrtc/api/rtpsenderinterface.h b/webrtc/api/rtpsenderinterface.h
index c3dd52f..776d01a 100644
--- a/webrtc/api/rtpsenderinterface.h
+++ b/webrtc/api/rtpsenderinterface.h
@@ -18,6 +18,7 @@
 
 #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"
@@ -51,6 +52,9 @@
 
   virtual void Stop() = 0;
 
+  virtual RtpParameters GetParameters() const = 0;
+  virtual bool SetParameters(const RtpParameters& parameters) = 0;
+
  protected:
   virtual ~RtpSenderInterface() {}
 };
@@ -66,6 +70,8 @@
 PROXY_METHOD1(void, set_stream_id, const std::string&)
 PROXY_CONSTMETHOD0(std::string, stream_id)
 PROXY_METHOD0(void, Stop)
+PROXY_CONSTMETHOD0(RtpParameters, GetParameters);
+PROXY_METHOD1(bool, SetParameters, const RtpParameters&)
 END_PROXY()
 
 }  // namespace webrtc