Bug 1503132 don't add audio output to offline streams r=padenot

Depends on D10173

Differential Revision: https://phabricator.services.mozilla.com/D10174

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

dom/media/webaudio/AudioDestinationNode.cpp

@@ -370,10 +370,13 @@ AudioDestinationNode::AudioDestinationNode(AudioContext* aContext,
     AudioNodeStream::EXTERNAL_OUTPUT;
   mStream = AudioNodeStream::Create(aContext, engine, flags, graph);
   mStream->AddMainThreadListener(this);
-  mStream->AddAudioOutput(&gWebAudioOutputKey);
 
-  if (!aIsOffline && aAllowedToStart) {
+  if (!aIsOffline) {
-    graph->NotifyWhenGraphStarted(mStream);
+    mStream->AddAudioOutput(&gWebAudioOutputKey);
+
+    if (aAllowedToStart) {
+      graph->NotifyWhenGraphStarted(mStream);
+    }
   }
 }