Emit dummy display item when recorded picture is empty

Source/platform/graphics/paint/CachedDisplayItem.h

Index: Source/platform/graphics/paint/CachedDisplayItem.h
diff --git a/Source/platform/graphics/paint/CachedDisplayItem.h b/Source/platform/graphics/paint/CachedDisplayItem.h
index cac02750ac7b6336777b2008a3db0dd028bf256d..23aa65f583af21d63660c94a44d9ed30fca281a2 100644
--- a/Source/platform/graphics/paint/CachedDisplayItem.h
+++ b/Source/platform/graphics/paint/CachedDisplayItem.h
@@ -28,6 +28,10 @@ private:
     // CachedDisplayItem is never replayed or appended to WebDisplayItemList.
     virtual void replay(GraphicsContext*) override final { ASSERT_NOT_REACHED(); }
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override final { ASSERT_NOT_REACHED(); }
+
+#ifndef NDEBUG
+    virtual const char* name() const override { return "Cached"; }
+#endif
 };
 
 } // namespace blink