Make ActiveScriptWrappable the GC mixin it is.

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

Index: third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.h
diff --git a/third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.h b/third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.h
index 8b355ba1af854166780918560ec63510c27eb731..f30942ba8673fa3294ec44da46110821f219ccff 100644
--- a/third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.h
+++ b/third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.h
@@ -140,6 +140,7 @@ private:
 
 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();