Bug 1400889 - Keep constant output channels in MediaStreamGraph. r=padenot

MozReview-Commit-ID: JlQdxU0aNOf

--HG--
extra : rebase_source : 25b6f223c2158a4eb9175731cdbb8521de4ce312

dom/media/MediaStreamGraph.cpp

@@ -3437,6 +3437,7 @@ MediaStreamGraphImpl::MediaStreamGraphImpl(GraphDriverType aDriverRequested,
   , mFarendObserverRef(nullptr)
 #endif
   , mSelfRef(this)
+  , mOutputChannels(std::min<uint32_t>(8, CubebUtils::MaxNumberOfChannels()))
 #ifdef DEBUG
   , mCanRunMessagesSynchronously(false)
 #endif

dom/media/MediaStreamGraphImpl.h

@@ -422,7 +422,7 @@ public:
 
   uint32_t AudioChannelCount() const
   {
-    return std::min<uint32_t>(8, CubebUtils::MaxNumberOfChannels());
+    return mOutputChannels;
   }
 
   double MediaTimeToSeconds(GraphTime aTime) const
@@ -815,6 +815,11 @@ private:
    */
   nsTArray<WindowAndStream> mWindowCaptureStreams;
 
+  /**
+   * Number of channels on output.
+   */
+  const uint32_t mOutputChannels;
+
 #ifdef DEBUG
   /**
    * Used to assert when AppendMessage() runs ControlMessages synchronously.