Use Timestamp to represent packet receive timestamps

Before this CL, timestamps of received packets were rounded
to the nearest millisecond and stored as int64_t. Due to the
rounding it sometimes happened that timestamps later in the
pipeline that are not rounded seem to occur even before the
video frame was received.

Change-Id: I92d8f3540b23baae2d4a1dc6a7cb3f58bcdaad18
Bug: webrtc:12722
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/216398
Reviewed-by: Chen Xing <chxg@google.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33916}

modules/audio_coding/neteq/red_payload_splitter.cc

@@ -139,7 +139,7 @@ bool RedPayloadSplitter::SplitRed(PacketList* packet_list) {
             /*rtp_timestamp=*/new_packet.timestamp,
             /*audio_level=*/absl::nullopt,
             /*absolute_capture_time=*/absl::nullopt,
-            /*receive_time_ms=*/red_packet.packet_info.receive_time_ms());
+            /*receive_time=*/red_packet.packet_info.receive_time());
         new_packets.push_front(std::move(new_packet));
         payload_ptr += payload_length;
       }