Store RtpPacketReceived::arrival_time as Timestamp.

Previously this value was rounded up to a millisecond value.
This change is complementary to another change:
https://webrtc-review.googlesource.com/c/src/+/216398

Bug: webrtc:12722
Change-Id: I0fd2baceb4608132615fb6ad241ec863e343edb1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/217521
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33928}
diff --git a/video/rtp_video_stream_receiver2.cc b/video/rtp_video_stream_receiver2.cc
index 5b282b4..9e68fa2 100644
--- a/video/rtp_video_stream_receiver2.cc
+++ b/video/rtp_video_stream_receiver2.cc
@@ -1145,7 +1145,7 @@
        << " with payload type: " << static_cast<int>(packet.PayloadType())
        << ", timestamp: " << packet.Timestamp()
        << ", sequence number: " << packet.SequenceNumber()
-       << ", arrival time: " << packet.arrival_time_ms();
+       << ", arrival time: " << ToString(packet.arrival_time());
     int32_t time_offset;
     if (packet.GetExtension<TransmissionOffset>(&time_offset)) {
       ss << ", toffset: " << time_offset;