message: Add OnDatagramAcked interface
  Cr-Commit-Position: refs/heads/master@{#28111}
diff: @@ -31,6 +31,13 @@ namespace webrtc {
  typedef int64_t DatagramId;
  +// TODO(mellem): Add receive timestamp.
  +struct DatagramAck {
  + // |datagram_id| is same as passed in
  + // DatagramTransportInterface::SendDatagram.
  + DatagramId datagram_id;
  +};
  +
  // All sink methods are called on network thread.
  class DatagramSinkInterface {
  public:
  @@ -41,8 +48,12 @@ class DatagramSinkInterface {
  // Called when datagram is actually sent (datragram can be delayed due
  // to congestion control or fusing). |datagram_id| is same as passed in
  - // QuicTransportInterface::SendDatagram.
  + // DatagramTransportInterface::SendDatagram.
  virtual void OnDatagramSent(DatagramId datagram_id) = 0;
  +
  + // Called when datagram is ACKed.
  + // TODO(sukhanov): Make pure virtual.
  + virtual void OnDatagramAcked(const DatagramAck& datagram_ack) {}
  };
  // Datagram transport allows to send and receive unreliable packets (datagrams)
  