Bug 1395593 - Don't query the maximum channel count in ::PlayAudio using cubeb. r=achronop a=ill-communication

MozReview-Commit-ID: 97ri5vhzsRF

dom/media/GraphDriver.h

@@ -445,6 +445,12 @@ public:
     return this;
   }
 
+  uint32_t OutputChannelCount()
+  {
+    MOZ_ASSERT(mOuputChannels != 0 && mOuputChannels <= 8);
+    return mOuputChannels;
+  }
+
   /* Enqueue a promise that is going to be resolved when a specific operation
    * occurs on the cubeb stream. */
   void EnqueueStreamAndPromiseForOperation(MediaStream* aStream,

dom/media/MediaStreamGraph.cpp

@@ -948,7 +948,8 @@ MediaStreamGraphImpl::PlayAudio(MediaStream* aStream)
 
     // Need unique id for stream & track - and we want it to match the inserter
     output.WriteTo(LATENCY_STREAM_ID(aStream, track->GetID()),
-                                     mMixer, AudioChannelCount(),
+                                     mMixer,
+                                     CurrentDriver()->AsAudioCallbackDriver()->OutputChannelCount(),
                                      mSampleRate);
   }
   return ticksWritten;