Reland "Avoid sending empty receiver reports with RtcpTransceiver"

This reverts commit 48420fa947cea4c618d51dc5f87908765a3a69db.

Reason for revert: downstream unittests adjusted

Original change's description:
> Revert "Avoid sending empty receiver reports with RtcpTransceiver"
>
> This reverts commit e5f1a3992e3bbfa0445b90f317576c8229524d74.
>
> Reason for revert: Speculative revert due to failing downstream unittest.
>
> Original change's description:
> > Avoid sending empty receiver reports with RtcpTransceiver
> >
> > Bug: None
> > Change-Id: Ia017c2df285febefb72ba88ba43366455bde5a78
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/222402
> > Reviewed-by: Per Kjellander <perkj@webrtc.org>
> > Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#34281}
>
> TBR=danilchap@webrtc.org,perkj@webrtc.org,webrtc-scoped@luci-project-accounts.iam.gserviceaccount.com
>
> Change-Id: I895317ad0381756e97e501a36d6440f83a68b6f8
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: None
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/222440
> Reviewed-by: Björn Terelius <terelius@webrtc.org>
> Commit-Queue: Björn Terelius <terelius@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#34284}

# Not skipping CQ checks because this is a reland.

Bug: None
Change-Id: I3481b9b12ddabaef7303ba80e9cd885930988caa
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/222600
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34291}

modules/rtp_rtcp/source/rtcp_transceiver_impl.cc

diff --git a/modules/rtp_rtcp/source/rtcp_transceiver_impl.cc b/modules/rtp_rtcp/source/rtcp_transceiver_impl.cc
index df67c07..db065e2 100644
--- a/modules/rtp_rtcp/source/rtcp_transceiver_impl.cc
+++ b/modules/rtp_rtcp/source/rtcp_transceiver_impl.cc
@@ -355,9 +355,12 @@
   rtcp::ReceiverReport receiver_report;
   receiver_report.SetSenderSsrc(sender_ssrc);
   receiver_report.SetReportBlocks(CreateReportBlocks(now));
-  sender->AppendPacket(receiver_report);
+  if (config_.rtcp_mode == RtcpMode::kCompound ||
+      !receiver_report.report_blocks().empty()) {
+    sender->AppendPacket(receiver_report);
+  }
 
-  if (!config_.cname.empty()) {
+  if (!config_.cname.empty() && !sender->IsEmpty()) {
     rtcp::Sdes sdes;
     bool added = sdes.AddCName(config_.feedback_ssrc, config_.cname);
     RTC_DCHECK(added) << "Failed to add cname " << config_.cname

modules/rtp_rtcp/source/rtcp_transceiver_impl_unittest.cc

diff --git a/modules/rtp_rtcp/source/rtcp_transceiver_impl_unittest.cc b/modules/rtp_rtcp/source/rtcp_transceiver_impl_unittest.cc
index 6af34a9..71b457b 100644
--- a/modules/rtp_rtcp/source/rtcp_transceiver_impl_unittest.cc
+++ b/modules/rtp_rtcp/source/rtcp_transceiver_impl_unittest.cc
@@ -38,6 +38,7 @@
 
 using ::testing::_;
 using ::testing::ElementsAre;
+using ::testing::NiceMock;
 using ::testing::Return;
 using ::testing::SizeIs;
 using ::testing::StrictMock;
@@ -392,6 +393,47 @@
   EXPECT_EQ(rtcp_parser.sdes()->chunks()[0].cname, config.cname);
 }
 
+TEST(RtcpTransceiverImplTest, AvoidsEmptyPacketsInReducedMode) {
+  MockTransport transport;
+  EXPECT_CALL(transport, SendRtcp).Times(0);
+  NiceMock<MockReceiveStatisticsProvider> receive_statistics;
+  SimulatedClock clock(0);
+
+  RtcpTransceiverConfig config = DefaultTestConfig();
+  config.clock = &clock;
+  config.outgoing_transport = &transport;
+  config.rtcp_mode = webrtc::RtcpMode::kReducedSize;
+  config.schedule_periodic_compound_packets = false;
+  config.receive_statistics = &receive_statistics;
+  RtcpTransceiverImpl rtcp_transceiver(config);
+
+  rtcp_transceiver.SendCompoundPacket();
+}
+
+TEST(RtcpTransceiverImplTest, AvoidsEmptyReceiverReportsInReducedMode) {
+  RtcpPacketParser rtcp_parser;
+  RtcpParserTransport transport(&rtcp_parser);
+  NiceMock<MockReceiveStatisticsProvider> receive_statistics;
+  SimulatedClock clock(0);
+
+  RtcpTransceiverConfig config = DefaultTestConfig();
+  config.clock = &clock;
+  config.outgoing_transport = &transport;
+  config.rtcp_mode = webrtc::RtcpMode::kReducedSize;
+  config.schedule_periodic_compound_packets = false;
+  config.receive_statistics = &receive_statistics;
+  // Set it to produce something (RRTR) in the "periodic" rtcp packets.
+  config.non_sender_rtt_measurement = true;
+  RtcpTransceiverImpl rtcp_transceiver(config);
+
+  // Rather than waiting for the right time to produce the periodic packet,
+  // trigger it manually.
+  rtcp_transceiver.SendCompoundPacket();
+
+  EXPECT_EQ(rtcp_parser.receiver_report()->num_packets(), 0);
+  EXPECT_GT(rtcp_parser.xr()->num_packets(), 0);
+}
+
 TEST(RtcpTransceiverImplTest, SendsNoRembInitially) {
   const uint32_t kSenderSsrc = 12345;
   SimulatedClock clock(0);