Fix unit for inbound RTP stat `lastPacketReceivedTimestamp` (s -> ms)

Both inbound RTP stats `estimatedPlayoutTimestamp` and
`lastPacketReceivedTimestamp` are surfaced to JS land as
`DOMHighResTimeStamp` - i.e., time values in milliseconds.
This CL fixes `lastPacketReceivedTimestamp` which is incorrectly
surfaced as time value in seconds.

Bug: webrtc:12605
Change-Id: I290103071cca3331d2a3066b6b6b9fcb4f4fd0af
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/212742
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33530}
diff --git a/pc/rtc_stats_collector.cc b/pc/rtc_stats_collector.cc
index 122ae9f..6f5f035 100644
--- a/pc/rtc_stats_collector.cc
+++ b/pc/rtc_stats_collector.cc
@@ -347,10 +347,8 @@
   // |fir_count|, |pli_count| and |sli_count| are only valid for video and are
   // purposefully left undefined for audio.
   if (voice_receiver_info.last_packet_received_timestamp_ms) {
-    inbound_audio->last_packet_received_timestamp =
-        static_cast<double>(
-            *voice_receiver_info.last_packet_received_timestamp_ms) /
-        rtc::kNumMillisecsPerSec;
+    inbound_audio->last_packet_received_timestamp = static_cast<double>(
+        *voice_receiver_info.last_packet_received_timestamp_ms);
   }
   if (voice_receiver_info.estimated_playout_ntp_timestamp_ms) {
     inbound_audio->estimated_playout_timestamp = static_cast<double>(
diff --git a/pc/rtc_stats_collector_unittest.cc b/pc/rtc_stats_collector_unittest.cc
index 6134846..1516bba 100644
--- a/pc/rtc_stats_collector_unittest.cc
+++ b/pc/rtc_stats_collector_unittest.cc
@@ -1864,7 +1864,7 @@
 
   // Set previously undefined values and "GetStats" again.
   voice_media_info.receivers[0].last_packet_received_timestamp_ms = 3000;
-  expected_audio.last_packet_received_timestamp = 3.0;
+  expected_audio.last_packet_received_timestamp = 3000.0;
   voice_media_info.receivers[0].estimated_playout_ntp_timestamp_ms = 4567;
   expected_audio.estimated_playout_timestamp = 4567;
   voice_media_channel->SetStats(voice_media_info);