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

Source/core/events/SecurityPolicyViolationEvent.h

Index: Source/core/events/SecurityPolicyViolationEvent.h
diff --git a/Source/core/events/SecurityPolicyViolationEvent.h b/Source/core/events/SecurityPolicyViolationEvent.h
index 5c52cab687bba13c1eb51a242985b5f2e4a81e63..efe2481272d2b03fd61a68b9c329ec9f3b6314d7 100644
--- a/Source/core/events/SecurityPolicyViolationEvent.h
+++ b/Source/core/events/SecurityPolicyViolationEvent.h
@@ -72,6 +72,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE { return EventNames::SecurityPolicyViolationEvent; }
 
+    virtual void trace(Visitor* visitor) OVERRIDE { Event::trace(visitor); }
+
 private:
     SecurityPolicyViolationEvent()
     {