Bug 1586387 - Don't append a device change message to a graph that is going away. r=padenot

Differential Revision: https://phabricator.services.mozilla.com/D48553

--HG--
extra : moz-landing-system : lando

dom/media/MediaTrackGraph.cpp

@@ -866,6 +866,15 @@ void MediaTrackGraphImpl::DeviceChanged() {
     MediaTrackGraphImpl* mGraphImpl;
   };
 
+  if (mMainThreadTrackCount == 0 && mMainThreadPortCount == 0) {
+    // This is a special case where the origin of this event cannot control the
+    // lifetime of the graph, because the graph is controling the lifetime of
+    // the AudioCallbackDriver where the event originated.
+    // We know the graph is soon going away, so there's no need to notify about
+    // this device change.
+    return;
+  }
+
   // Reset the latency, it will get fetched again next time it's queried.
   MOZ_ASSERT(NS_IsMainThread());
   mAudioOutputLatency = 0.0;