message: Add a clock to be used for getting the NTP time in RtcpTransceiverConfig.
  Note: google3 needs to set this clock before we can start using it.
  Cr-Commit-Position: refs/heads/master@{#33653}
diff: #include "api/task_queue/task_queue_base.h"
  #include "api/video/video_bitrate_allocation.h"
  #include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
  +#include "system_wrappers/include/clock.h"
  #include "system_wrappers/include/ntp_time.h"
  namespace webrtc {
  @@ -61,6 +62,9 @@ struct RtcpTransceiverConfig {
  // Maximum packet size outgoing transport accepts.
  size_t max_packet_size = 1200;
  + // The clock to use when querying for the NTP time. Should be set.
  + Clock* clock = nullptr;
  +
  // Transport to send rtcp packets to. Should be set.
  Transport* outgoing_transport = nullptr;
  