Fix possible crash TransceiverStateSurfacer::Initialize() crash.

It may be possible that a peer connection is closed, garbage collected
and that any local track adapters associated with it is destroyed before
the setLocalDescription/setRemoteDescription observer callback is
invoked on the signaling thread due to webrtc::PeerConnection invoking
the observer with an asynchronous delay.

TransceiverStateSurfacer::Initialize() will crash if local track
adapters are missing. This CL makes
WebRtcSet[Local/Remote]DescriptionObserverHandler not surface any
transceivers if the webrtc-layer peer connection is closed. This should
be fine, because the spec says to abort the SLD/SRD steps if the PC has
been closed.

This is a speculative fix for https://crbug.com/897251. The stack trace
produced by the new unittest before the fix is the same as that bug, but
the cause of the referenced bug being that the PC was closed and GC'd,
as assumed by this CL, has not been confirmed.

  NOTE: This CL contains just the fix.
  https://chromium-review.googlesource.com/c/chromium/src/+/1309792
  contains tests too but we broke up the CL to allow this to land
  before a dependent testing related CL lands.

TBR=guidou@chromium.org

Bug: 897251
Change-Id: Ibf621c45fd5a683d847596edeea1f88018783289
Reviewed-on: https://chromium-review.googlesource.com/c/1311919Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604548}

content/renderer/media/webrtc/webrtc_set_description_observer.cc

@@ -53,12 +53,19 @@ void WebRtcSetDescriptionObserverHandlerImpl::OnSetDescriptionComplete(
   std::vector<rtc::scoped_refptr<webrtc::RtpTransceiverInterface>>
       receiver_only_transceivers;
   std::vector<rtc::scoped_refptr<webrtc::RtpTransceiverInterface>> transceivers;
-  if (surface_receivers_only_) {
-    for (const auto& receiver : pc_->GetReceivers()) {
-      transceivers.push_back(new SurfaceReceiverStateOnly(receiver));
+  // Only surface transceiver/receiver states if the peer connection is not
+  // closed. If the peer connection is closed, the peer connection handler may
+  // have been destroyed along with any track adapters that
+  // TransceiverStateSurfacer assumes exist. This is treated as a special case
+  // due to https://crbug.com/897251.
+  if (pc_->signaling_state() != webrtc::PeerConnectionInterface::kClosed) {
+    if (surface_receivers_only_) {
+      for (const auto& receiver : pc_->GetReceivers()) {
+        transceivers.push_back(new SurfaceReceiverStateOnly(receiver));
+      }
+    } else {
+      transceivers = pc_->GetTransceivers();
     }
-  } else {
-    transceivers = pc_->GetTransceivers();
   }
   TransceiverStateSurfacer transceiver_state_surfacer(main_task_runner_,
                                                       signaling_task_runner_);