Propagate packet pacing information to SenTimeHistory.

In order to not make this CL too large I have broken it down into at least two steps. In this CL we only propagate the pacing information part of the way:

webrtc::PacedSender::Process                        <--- propagate from here
webrtc::PacedSender::SendPacket
webrtc::PacketRouter::TimeToSendPacket
webrtc::ModuleRtpRtcpImpl::TimeToSendPacket         <--- to here
webrtc::RTPSender::TimeToSendPacket
webrtc::RTPSender::PrepareAndSendPacket
webrtc::RTPSender::AddPacketToTransportFeedback
webrtc::TransportFeedbackAdapter::AddPacket
webrtc::SendTimeHistory::AddAndRemoveOld            <--- goal is to propagte it here

BUG=webrtc:6822

Review-Url: https://codereview.webrtc.org/2628563003
Cr-Commit-Position: refs/heads/master@{#16664}

webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h

@@ -251,7 +251,7 @@ struct PacketInfo {
                    -1,
                    sequence_number,
                    0,
-                   kNotAProbe) {}
+                   PacedPacketInfo::kNotAProbe) {}
 
   PacketInfo(int64_t arrival_time_ms,
              int64_t send_time_ms,
@@ -278,8 +278,6 @@ struct PacketInfo {
         payload_size(payload_size),
         probe_cluster_id(probe_cluster_id) {}
 
-  static constexpr int kNotAProbe = -1;
-
   // Time corresponding to when this object was created.
   int64_t creation_time_ms;
   // Time corresponding to when the packet was received. Timestamped with the
@@ -294,6 +292,7 @@ struct PacketInfo {
   // Size of the packet excluding RTP headers.
   size_t payload_size;
   // Which probing cluster this packets belongs to.
+  // TODO(philipel): replace this with pacing information when it is available.
   int probe_cluster_id;
 };