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

Source/modules/mediastream/RTCIceCandidateEvent.h

Index: Source/modules/mediastream/RTCIceCandidateEvent.h
diff --git a/Source/modules/mediastream/RTCIceCandidateEvent.h b/Source/modules/mediastream/RTCIceCandidateEvent.h
index 5129ec7aee84346fca39d30dee87402f10dbb722..03ec5b9d94b7b281a7afad50920d9fc7f9526cf8 100644
--- a/Source/modules/mediastream/RTCIceCandidateEvent.h
+++ b/Source/modules/mediastream/RTCIceCandidateEvent.h
@@ -42,6 +42,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     RTCIceCandidateEvent();
     RTCIceCandidateEvent(bool canBubble, bool cancelable, PassRefPtr<RTCIceCandidate>);