Emit dummy display item when recorded picture is empty

Source/platform/graphics/paint/CachedDisplayItem.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CachedDisplayItem_h
 #define CachedDisplayItem_h
 
 #include "platform/graphics/paint/DisplayItem.h"
 #include "wtf/Assertions.h"
 
@@ skipping 10 matching lines @@
     }
 
     virtual bool isCached() const { return true; }
 
 private:
     CachedDisplayItem(DisplayItemClient client, Type type) : DisplayItem(client, type) { }
 
     // 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
 
 #endif // CachedDisplayItem_h