Bug 938686 - Make OggReader::DownmixToStereo public static. r=rillian

We want this to be available to the WebM reader as well.

content/media/ogg/OggReader.h

@@ -78,6 +78,12 @@ public:
   virtual nsresult Seek(int64_t aTime, int64_t aStartTime, int64_t aEndTime, int64_t aCurrentTime);
   virtual nsresult GetBuffered(dom::TimeRanges* aBuffered, int64_t aStartTime);
 
+  // Downmix multichannel Audio samples to Stereo.
+  // It is used from Vorbis and Opus decoders.
+  // Input are the buffer contains multichannel data,
+  // the number of channels and the number of frames.
+  static void DownmixToStereo(nsAutoArrayPtr<AudioDataValue>& buffer,
+                     uint32_t& channel, int32_t frames);
 private:
   // This monitor should be taken when reading or writing to mIsChained.
   ReentrantMonitor mMonitor;
@@ -223,12 +229,6 @@ private:
   // audio queue.
   nsresult DecodeOpus(ogg_packet* aPacket);
 
-  // Downmix multichannel Audio samples to Stereo.
-  // It is used from Vorbis and Opus decoders.
-  // Input are the buffer contains multichannel data,
-  // the number of channels and the number of frames.
-  void DownmixToStereo(nsAutoArrayPtr<AudioDataValue>& buffer,
-                     uint32_t& channel, int32_t frames);
 
   // Decodes a packet of Theora data, and inserts its frame into the
   // video queue. May return NS_ERROR_OUT_OF_MEMORY. Caller must have obtained