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

Source/core/html/canvas/WebGLContextEvent.h

Index: Source/core/html/canvas/WebGLContextEvent.h
diff --git a/Source/core/html/canvas/WebGLContextEvent.h b/Source/core/html/canvas/WebGLContextEvent.h
index acba861b8ae96f9774c0fae36f29ae2c92d6448c..4326054855205e77843a8e35640debf07c899bf5 100644
--- a/Source/core/html/canvas/WebGLContextEvent.h
+++ b/Source/core/html/canvas/WebGLContextEvent.h
@@ -56,6 +56,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     WebGLContextEvent();
     WebGLContextEvent(const AtomicString& type, bool canBubble, bool cancelable, const String& statusMessage);