Make Event RefCountedGarbageCollected and implement trace() methods to the Event hierarcy

Source/modules/webaudio/AudioProcessingEvent.h

Index: Source/modules/webaudio/AudioProcessingEvent.h
diff --git a/Source/modules/webaudio/AudioProcessingEvent.h b/Source/modules/webaudio/AudioProcessingEvent.h
index 0f83f3eb928dd1eb1ab3f1ff39b2af98a814455e..23f61f0906be7258a35dc8e58884f0e29b5943e7 100644
--- a/Source/modules/webaudio/AudioProcessingEvent.h
+++ b/Source/modules/webaudio/AudioProcessingEvent.h
@@ -46,6 +46,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     AudioProcessingEvent();
     AudioProcessingEvent(PassRefPtr<AudioBuffer> inputBuffer, PassRefPtr<AudioBuffer> outputBuffer);