Make ActiveScriptWrappable the GC mixin it is.

third_party/WebKit/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 122 matching lines @@
     // Compute playback rate (k-rate) by incorporating the sample rate
     // conversion factor, and the value of playbackRate and detune AudioParams.
     double computePlaybackRate();
 
     // The minimum playbackRate value ever used for this source.
     double m_minPlaybackRate;
 };
 
 class AudioBufferSourceNode final : public AudioScheduledSourceNode {
     DEFINE_WRAPPERTYPEINFO();
+    USING_GARBAGE_COLLECTED_MIXIN(AudioBufferSourceNode);

[hongchan, 2016/05/10 16:58:28]

AudioScheduledSourceNode already has mixin. Is this necessary?

If this is necessary, OsciallatorNode also should be taken care of.

[sof, 2016/05/10 19:02:01]

Thanks for catching that, it is unnecessary as it only extends its only
(leftmost) derived class.

Let's remove it, see https://codereview.chromium.org/1969453003/

 public:
     static AudioBufferSourceNode* create(AbstractAudioContext&, float sampleRate);
     DECLARE_VIRTUAL_TRACE();
     AudioBufferSourceHandler& audioBufferSourceHandler() const;
 
     AudioBuffer* buffer() const;
     void setBuffer(AudioBuffer*, ExceptionState&);
     AudioParam* playbackRate() const;
     AudioParam* detune() const;
     bool loop() const;
@@ skipping 11 matching lines @@
 private:
     AudioBufferSourceNode(AbstractAudioContext&, float sampleRate);
 
     Member<AudioParam> m_playbackRate;
     Member<AudioParam> m_detune;
 };
 
 } // namespace blink
 
 #endif // AudioBufferSourceNode_h