Add TransportFeedbackPacketLossTracker.

webrtc/modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h

Index: webrtc/modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h b/webrtc/modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h
index 03a1c4f728ac217d4cce1c0d03add9cb1bef7eb7..e8634c8139e4d3da93b0b4a9cc2a75835f96d494 100644
--- a/webrtc/modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h
+++ b/webrtc/modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h
@@ -21,9 +21,31 @@
 
 namespace webrtc {
 namespace rtcp {
+
+class TransportFeedbackInterface {
+ public:
+  virtual ~TransportFeedbackInterface() = default;
+  enum class StatusSymbol {
+    kNotReceived,
+    kReceivedSmallDelta,
+    kReceivedLargeDelta,
+  };
+  virtual void SetBase(uint16_t base_sequence, int64_t ref_timestamp_us) = 0;
+  virtual void SetFeedbackSequenceNumber(uint8_t feedback_sequence) = 0;
+  virtual bool AddReceivedPacket(uint16_t sequence_number,
+                                 int64_t timestamp_us) = 0;
+  virtual uint16_t GetBaseSequence() const = 0;
+  virtual std::vector<StatusSymbol> GetStatusVector() const = 0;
+  virtual std::vector<int16_t> GetReceiveDeltas() const = 0;
+  virtual int64_t GetBaseTimeUs() const = 0;
+  virtual std::vector<int64_t> GetReceiveDeltasUs() const = 0;
+};
+
 class CommonHeader;
 
-class TransportFeedback : public Rtpfb {
+// TODO(minyue): Update PacketCounter so that TransportFeedback can become a
+// final class.
+class TransportFeedback : public Rtpfb, public TransportFeedbackInterface {
  public:
   class PacketStatusChunk;
   // TODO(sprang): IANA reg?
@@ -34,27 +56,23 @@ class TransportFeedback : public Rtpfb {
   TransportFeedback();
   ~TransportFeedback() override;
 
-  void SetBase(uint16_t base_sequence,     // Seq# of first packet in this msg.
-               int64_t ref_timestamp_us);  // Reference timestamp for this msg.
-  void SetFeedbackSequenceNumber(uint8_t feedback_sequence);
+  void SetBase(uint16_t base_sequence,    // Seq# of first packet in this msg.
+               int64_t ref_timestamp_us)  // Reference timestamp for this msg.
+      override;
+  void SetFeedbackSequenceNumber(uint8_t feedback_sequence) override;
   // NOTE: This method requires increasing sequence numbers (excepting wraps).
-  bool AddReceivedPacket(uint16_t sequence_number, int64_t timestamp_us);
-
-  enum class StatusSymbol {
-    kNotReceived,
-    kReceivedSmallDelta,
-    kReceivedLargeDelta,
-  };
+  bool AddReceivedPacket(uint16_t sequence_number,
+                         int64_t timestamp_us) override;
 
-  uint16_t GetBaseSequence() const;
-  std::vector<TransportFeedback::StatusSymbol> GetStatusVector() const;
-  std::vector<int16_t> GetReceiveDeltas() const;
+  uint16_t GetBaseSequence() const override;
+  std::vector<StatusSymbol> GetStatusVector() const override;
+  std::vector<int16_t> GetReceiveDeltas() const override;
 
   // Get the reference time in microseconds, including any precision loss.
-  int64_t GetBaseTimeUs() const;
+  int64_t GetBaseTimeUs() const override;
   // Convenience method for getting all deltas as microseconds. The first delta
   // is relative the base time.
-  std::vector<int64_t> GetReceiveDeltasUs() const;
+  std::vector<int64_t> GetReceiveDeltasUs() const override;
 
   bool Parse(const CommonHeader& packet);
   static std::unique_ptr<TransportFeedback> ParseFrom(const uint8_t* buffer,