Remove obsolete DCHECK in remote_audio_source.cc.

When fixing so that RemoteAudioSource does not end the track just
because the audio channel is gone in Unified Plan[1], this made it
possible for ~PeerConnection to delete all objects, including deleting
the MediaStreamTrack and its RemoteAudioSource, when all tracks are not
in an ended state.

In a real application or Chromium, the PeerConnection would not be
destroyed prior to closing and not hit this DCHECK. But in upstream
dependent projects' unit tests, it would be possible for ref counted
tracks to be destroyed when the track are still kLive, and as a
side-effect hit this DCHECK.

sinks_ is just a list of raw pointers, and whether or not we have done
sinks_.clear() prior to destruction is irrelevant going forward.

[1] https://webrtc-review.googlesource.com/c/src/+/214136

Bug: chromium:1121454
Change-Id: If6cf3dffcd3cb47d46694755b5dc45fa381285fc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215226
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33739}
diff --git a/pc/remote_audio_source.cc b/pc/remote_audio_source.cc
index 848fba3..9e65f67 100644
--- a/pc/remote_audio_source.cc
+++ b/pc/remote_audio_source.cc
@@ -63,7 +63,10 @@
 RemoteAudioSource::~RemoteAudioSource() {
   RTC_DCHECK(main_thread_->IsCurrent());
   RTC_DCHECK(audio_observers_.empty());
-  RTC_DCHECK(sinks_.empty());
+  if (!sinks_.empty()) {
+    RTC_LOG(LS_WARNING)
+        << "RemoteAudioSource destroyed while sinks_ is non-empty.";
+  }
 }
 
 void RemoteAudioSource::Start(cricket::VoiceMediaChannel* media_channel,