Don't remove empty display item pairs that draw content

Source/platform/graphics/paint/DrawingDisplayItem.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 DrawingDisplayItem_h
 #define DrawingDisplayItem_h
 
 #include "platform/PlatformExport.h"
 #include "platform/geometry/FloatPoint.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "third_party/skia/include/core/SkPicture.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
 
 class PLATFORM_EXPORT DrawingDisplayItem : public DisplayItem {
 public:
     static PassOwnPtr<DrawingDisplayItem> create(DisplayItemClient client, Type type, PassRefPtr<const SkPicture> picture)
     {
         return adoptPtr(new DrawingDisplayItem(client, type, picture));
     }
 
     virtual void replay(GraphicsContext*);
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
+    virtual bool drawsContent() const override;
 
     PassRefPtr<const SkPicture> picture() const { return m_picture; }
 
 protected:
     DrawingDisplayItem(DisplayItemClient client, Type type, PassRefPtr<const SkPicture> picture)
         : DisplayItem(client, type), m_picture(picture)
     {
         ASSERT(m_picture);
         ASSERT(isDrawingType(type));
     }
 
 private:
 #ifndef NDEBUG
     virtual void dumpPropertiesAsDebugString(WTF::StringBuilder&) const override;
 #endif
 
     RefPtr<const SkPicture> m_picture;
 };
 
 } // namespace blink
 
 #endif // DrawingDisplayItem_h