Add a FlexfecReceiver class.

This class is split in interface/implementation classes, since it
will be referenced from the Call level. Its purpose is to interface
the erasure code decoder with a new class FlexfecReceiveStream
(for received packets), as well as with the main RTP pipeline (for
recovered packets).

BUG=webrtc:5654

Review-Url: https://codereview.webrtc.org/2392663006
Cr-Commit-Position: refs/heads/master@{#14594}

webrtc/modules/rtp_rtcp/include/flexfec_receiver.h

@@ -0,0 +1,50 @@
+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_MODULES_RTP_RTCP_INCLUDE_FLEXFEC_RECEIVER_H_
+#define WEBRTC_MODULES_RTP_RTCP_INCLUDE_FLEXFEC_RECEIVER_H_
+
+#include <memory>
+
+#include "webrtc/base/basictypes.h"
+#include "webrtc/modules/rtp_rtcp/include/fec_receiver.h"
+
+namespace webrtc {
+
+// Callback interface for packets recovered by FlexFEC.
+class RecoveredPacketReceiver {
+ public:
+  virtual bool OnRecoveredPacket(const uint8_t* packet, size_t length) = 0;
+
+ protected:
+  virtual ~RecoveredPacketReceiver() = default;
+};
+
+class FlexfecReceiver {
+ public:
+  static std::unique_ptr<FlexfecReceiver> Create(
+      uint32_t flexfec_ssrc,
+      uint32_t protected_media_ssrc,
+      RecoveredPacketReceiver* callback);
+  virtual ~FlexfecReceiver();
+
+  // Inserts a received packet (can be either media or FlexFEC) into the
+  // internal buffer, and sends the received packets to the erasure code.
+  // All newly recovered packets are sent back through the callback.
+  virtual bool AddAndProcessReceivedPacket(const uint8_t* packet,
+                                           size_t packet_length) = 0;
+
+  // Returns a counter describing the added and recovered packets.
+  virtual FecPacketCounter GetPacketCounter() const = 0;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_RTP_RTCP_INCLUDE_FLEXFEC_RECEIVER_H_