Reland Issue 2061423003: Refactor NACK bitrate allocation

This is a reland of https://codereview.webrtc.org/2061423003/
Which was reverted in https://codereview.webrtc.org/2131913003/

The reason for the revert was that some upstream code used
RtpSender::SetTargetBitrate(). I've added that back as a no-op until we
it's been brought up to date.

TBR=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/2131313002 .

Cr-Commit-Position: refs/heads/master@{#13418}

webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h

@@ -285,8 +285,6 @@ class ModuleRtpRtcpImpl : public RtpRtcp {
   // Send a request for a keyframe.
   int32_t RequestKeyFrame() override;
 
-  void SetTargetSendBitrate(uint32_t bitrate_bps) override;
-
   void SetGenericFECStatus(bool enable,
                            uint8_t payload_type_red,
                            uint8_t payload_type_fec) override;