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

Source/modules/speech/SpeechSynthesisEvent.h

Index: Source/modules/speech/SpeechSynthesisEvent.h
diff --git a/Source/modules/speech/SpeechSynthesisEvent.h b/Source/modules/speech/SpeechSynthesisEvent.h
index 6ac7495a1c8520fb972c1af27d29507810e30e98..1e290a6d5a46ddd00a6f64c3060002d13197c27b 100644
--- a/Source/modules/speech/SpeechSynthesisEvent.h
+++ b/Source/modules/speech/SpeechSynthesisEvent.h
@@ -42,6 +42,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE { return EventNames::SpeechSynthesisEvent; }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     SpeechSynthesisEvent();
     SpeechSynthesisEvent(const AtomicString& type, unsigned long charIndex, float elapsedTime, const String& name);