Add RemoteEstimatorProxy for capturing receive times

For use when send-side bandwidth estimation is enabled.

Receive times need to be captured, buffered and then sent using
TransportFeedback RTCP messaged back to the send side.

BUG=webrtc:4173

Review URL: https://codereview.webrtc.org/1290813008

Cr-Commit-Position: refs/heads/master@{#9898}

webrtc/modules/pacing/include/packet_router.h

@@ -19,6 +19,7 @@
 #include "webrtc/base/thread_annotations.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/pacing/include/paced_sender.h"
+#include "webrtc/modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h"
 
 namespace webrtc {
 
@@ -45,6 +46,9 @@ class PacketRouter : public PacedSender::Callback {
   void SetTransportWideSequenceNumber(uint16_t sequence_number);
   uint16_t AllocateSequenceNumber();
 
+  // Send transport feedback packet to send-side.
+  virtual bool SendFeedback(rtcp::TransportFeedback* packet);
+
  private:
   rtc::CriticalSection modules_lock_;
   // Map from ssrc to sending rtp module.