Set channelCount and maxChannelCount correctly for OfflineAudioContext.

third_party/WebKit/Source/modules/webaudio/OfflineAudioDestinationNode.h

 /*
  * Copyright (C) 2011, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 35 matching lines @@
     // AudioHandler
     void dispose() override;
     void initialize() override;
     void uninitialize() override;
 
     OfflineAudioContext* context() const final;
 
     // AudioDestinationHandler
     void startRendering() override;
     void stopRendering() override;
+    unsigned long maxChannelCount() const override;
 
     float sampleRate()  const override { return m_renderTarget->sampleRate(); }
 
     size_t renderQuantumFrames() const { return renderQuantumSize; }
 
     WebThread* offlineRenderThread();
 
 private:
     OfflineAudioDestinationHandler(AudioNode&, AudioBuffer* renderTarget);
 
@@ skipping 51 matching lines @@
 public:
     static OfflineAudioDestinationNode* create(AbstractAudioContext*, AudioBuffer* renderTarget);
 
 private:
     OfflineAudioDestinationNode(AbstractAudioContext&, AudioBuffer* renderTarget);
 };
 
 } // namespace blink
 
 #endif // OfflineAudioDestinationNode_h