Change Channel::GetRtpRtcp to return only RtpRtcp, not RtpReceiver.

For use in AudiReceiveStream, introduce a new method GetSyncInfo. This
change is analogous to https://webrtc-review.googlesource.com/91123,
doing the same for RtpVideoStreamReceiver. It's a preparation for
bypassing the RtpReceiver class.

Bug: webrtc:7135
Change-Id: I87c1c6f0a1f28b0baebe07c4181f6f0427afa314
Reviewed-on: https://webrtc-review.googlesource.com/93022
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24228}
diff --git a/audio/audio_send_stream_unittest.cc b/audio/audio_send_stream_unittest.cc
index 2341d4e..dba1f5c 100644
--- a/audio/audio_send_stream_unittest.cc
+++ b/audio/audio_send_stream_unittest.cc
@@ -189,12 +189,9 @@
   void SetupDefaultChannelProxy(bool audio_bwe_enabled) {
     EXPECT_TRUE(channel_proxy_ == nullptr);
     channel_proxy_ = new testing::StrictMock<MockVoEChannelProxy>();
-    EXPECT_CALL(*channel_proxy_, GetRtpRtcp(_, _))
-        .WillRepeatedly(Invoke(
-            [this](RtpRtcp** rtp_rtcp_module, RtpReceiver** rtp_receiver) {
-              *rtp_rtcp_module = &this->rtp_rtcp_;
-              *rtp_receiver = nullptr;  // Not deemed necessary for tests yet.
-            }));
+    EXPECT_CALL(*channel_proxy_, GetRtpRtcp()).WillRepeatedly(Invoke([this]() {
+      return &this->rtp_rtcp_;
+    }));
     EXPECT_CALL(*channel_proxy_, SetRTCPStatus(true)).Times(1);
     EXPECT_CALL(*channel_proxy_, SetLocalSSRC(kSsrc)).Times(1);
     EXPECT_CALL(*channel_proxy_, SetRTCP_CNAME(StrEq(kCName))).Times(1);