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

Source/core/events/BeforeTextInsertedEvent.h

Index: Source/core/events/BeforeTextInsertedEvent.h
diff --git a/Source/core/events/BeforeTextInsertedEvent.h b/Source/core/events/BeforeTextInsertedEvent.h
index 44f0434fa190106fde086d8ba67bf7a6f86dc2e2..fe0f3f7c27c867b48da8abc2fd8d5bf50688fba8 100644
--- a/Source/core/events/BeforeTextInsertedEvent.h
+++ b/Source/core/events/BeforeTextInsertedEvent.h
@@ -45,6 +45,8 @@ public:
     const String& text() const { return m_text; }
     void setText(const String& s) { m_text = s; }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     explicit BeforeTextInsertedEvent(const String&);