bug 1228484 throw instead of capturing a second stream to a different graph r=roc

--HG--
extra : rebase_source : d1795f2afc413612666a5eee62d27d68e2337407

dom/html/HTMLMediaElement.cpp

@@ -1883,6 +1883,11 @@ HTMLMediaElement::CaptureStreamInternal(bool aFinishWhenEnded,
     aGraph = MediaStreamGraph::GetInstance(graphDriverType, mAudioChannel);
   }
 
+  if (!mOutputStreams.IsEmpty() &&
+      aGraph != mOutputStreams[0].mStream->GetInputStream()->Graph()) {
+    return nullptr;
+  }
+
   OutputMediaStream* out = mOutputStreams.AppendElement();
   out->mStream = DOMMediaStream::CreateTrackUnionStream(window, aGraph);
   RefPtr<nsIPrincipal> principal = GetCurrentPrincipal();