Copied from upstream:
https://hg.mozilla.org/releases/mozilla-esr38/raw-rev/9719b71d72dd

# HG changeset patch
# User Byron Campen [:bwc] <docfaraday@gmail.com>
# Date 1454100887 21600
# Node ID 9719b71d72dd2a3c5ee12ace156af2a63d9595ac
# Parent  b68673d974a10f65390f80b36d4307eb31e44669
Bug 1234578 - Assert if PCM is destroyed improperly. r=rjesup, a=sylvestre

diff --git a/media/webrtc/signaling/src/peerconnection/PeerConnectionMedia.cpp b/media/webrtc/signaling/src/peerconnection/PeerConnectionMedia.cpp
--- a/media/webrtc/signaling/src/peerconnection/PeerConnectionMedia.cpp
+++ b/media/webrtc/signaling/src/peerconnection/PeerConnectionMedia.cpp
@@ -712,16 +712,18 @@ PeerConnectionMedia::SelfDestruct_m()
 {
   CSFLogDebug(logTag, "%s: ", __FUNCTION__);
 
   ASSERT_ON_THREAD(mMainThread);
 
   mLocalSourceStreams.Clear();
   mRemoteSourceStreams.Clear();
 
+  mMainThread = nullptr;
+
   // Final self-destruct.
   this->Release();
 }
 
 void
 PeerConnectionMedia::ShutdownMediaTransport_s()
 {
   ASSERT_ON_THREAD(mSTSThread);
diff --git a/media/webrtc/signaling/src/peerconnection/PeerConnectionMedia.h b/media/webrtc/signaling/src/peerconnection/PeerConnectionMedia.h
--- a/media/webrtc/signaling/src/peerconnection/PeerConnectionMedia.h
+++ b/media/webrtc/signaling/src/peerconnection/PeerConnectionMedia.h
@@ -210,17 +210,20 @@ class RemoteSourceStreamInfo : public So
   std::vector<std::string> mTrackIdMap;
 
   // True iff SetPullEnabled(true) has been called on the DOMMediaStream. This
   // happens when offer/answer concludes.
   bool mReceiving;
 };
 
 class PeerConnectionMedia : public sigslot::has_slots<> {
-  ~PeerConnectionMedia() {}
+  ~PeerConnectionMedia()
+  {
+    MOZ_RELEASE_ASSERT(!mMainThread);
+  }
 
  public:
   explicit PeerConnectionMedia(PeerConnectionImpl *parent);
 
   PeerConnectionImpl* GetPC() { return mParent; }
   nsresult Init(const std::vector<NrIceStunServer>& stun_servers,
                 const std::vector<NrIceTurnServer>& turn_servers);
   // WARNING: This destroys the object!