Support nullable buffer for AudioBufferSourceNode.

Source/modules/webaudio/AudioBufferSourceNode.h

 /*
  * Copyright (C) 2010, 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 88 matching lines @@
 
     // Returns true on success.
     bool renderFromBuffer(AudioBus*, unsigned destinationFrameOffset, size_t numberOfFrames);
 
     // Render silence starting from "index" frame in AudioBus.
     inline bool renderSilenceAndFinishIfNotLooping(AudioBus*, unsigned index, size_t framesToProcess);
 
     // m_buffer holds the sample data which this node outputs.
     RefPtr<AudioBuffer> m_buffer;
 
+    // Used for setBuffer() called with null.
+    RefPtr<AudioBuffer> m_nullableBuffer;
+
     // Pointers for the buffer and destination.
     OwnPtr<const float*[]> m_sourceChannels;
     OwnPtr<float*[]> m_destinationChannels;
 
     // Used for the "gain" and "playbackRate" attributes.
     RefPtr<AudioParam> m_gain;
     RefPtr<AudioParam> m_playbackRate;
 
     // If m_isLooping is false, then this node will be done playing and become inactive after it reaches the end of the sample data in the buffer.
     // If true, it will wrap around to the start of the buffer each time it reaches the end.
@@ skipping 22 matching lines @@
     // the pitch rate. We manually manage ref-counting because we want to use RefTypeConnection.
     PannerNode* m_pannerNode;
 
     // This synchronizes process() with setBuffer() which can cause dynamic channel count changes.
     mutable Mutex m_processLock;
 };
 
 } // namespace WebCore
 
 #endif // AudioBufferSourceNode_h