Bug 573202 - annotate nsDisplayCanvasFocus for trace-refcnt. r=roc

layout/generic/nsCanvasFrame.cpp

@@ -292,6 +292,10 @@ public:
   nsDisplayCanvasFocus(nsCanvasFrame *aFrame)
     : nsDisplayItem(aFrame)
   {
+    MOZ_COUNT_CTOR(nsDisplayCanvasFocus);
+  }
+  virtual ~nsDisplayCanvasFocus() {
+    MOZ_COUNT_DTOR(nsDisplayCanvasFocus);
   }
 
   virtual nsRect GetBounds(nsDisplayListBuilder* aBuilder)