Fix unspecified time origin for lastPacketReceivedTimestamp

`RTCInboundRtpStreamStats.lastPacketReceivedTimestamp` must be a time
value in milliseconds with Unix epoch as time origin (see
bugs.webrtc.org/12605#c4).

This change fixes both audio and video `RTCInboundRtpStreamStats` stats.

Tested: verified from chrome://webrtc-internals during an appr.tc call

Bug: webrtc:12605
Change-Id: I68157fcf01a5933f3d4e5d3918b4a9d3fbd64f16
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/212865
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33547}

modules/rtp_rtcp/source/receive_statistics_impl.h

@@ -73,6 +73,8 @@ class StreamStatisticianImpl : public StreamStatisticianImplInterface {
 
   const uint32_t ssrc_;
   Clock* const clock_;
+  // Delta used to map internal timestamps to Unix epoch ones.
+  const int64_t delta_internal_unix_epoch_ms_;
   RateStatistics incoming_bitrate_;
   // In number of packets or sequence numbers.
   int max_reordering_threshold_;