bug 1207003 coallesce external output chunks when silent r=padenot

AppendAndConsumeChunk() will always append a new chunk, but AppendNullData()
will use an existing null chunk if possible.

--HG--
extra : rebase_source : b00a131433e5548d2d5bb90303cf4d2e9568419b

dom/media/webaudio/AudioNodeStream.cpp

@@ -604,7 +604,7 @@ AudioNodeStream::AdvanceOutputSegment()
 
   AudioSegment* segment = track->Get<AudioSegment>();
 
-  if (mFlags & EXTERNAL_OUTPUT) {
+  if (mFlags & EXTERNAL_OUTPUT && !mLastChunks[0].IsNull()) {
     segment->AppendAndConsumeChunk(mLastChunks[0].AsMutableChunk());
   } else {
     segment->AppendNullData(mLastChunks[0].GetDuration());