Avoid empty display item pairs

Source/platform/graphics/paint/DisplayItem.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 DisplayItem_h
 #define DisplayItem_h
 
 #include "platform/PlatformExport.h"
 #include "platform/graphics/paint/DisplayItemClient.h"
 #include "wtf/Assertions.h"
@@ skipping 167 matching lines @@
     DEFINE_PAINT_PHASE_CONVERSION_METHOD(ClipLayerFragment)
     DEFINE_PAINT_PHASE_CONVERSION_METHOD(ClipBox)
     DEFINE_PAINT_PHASE_CONVERSION_METHOD(ClipColumnBounds)
 
     DEFINE_PAIRED_CATEGORY_METHODS(FloatClip, floatClip)
     DEFINE_PAINT_PHASE_CONVERSION_METHOD(FloatClip)
 
     DEFINE_PAIRED_CATEGORY_METHODS(Scroll, scroll)
     DEFINE_PAINT_PHASE_CONVERSION_METHOD(Scroll)
 
+    virtual bool isBegin() const { return false; }
+    virtual bool isEnd() const { return false; }
+
+#if ENABLE(ASSERT)
+    virtual bool isEndAndPairedWith(const DisplayItem& other) const { return false; }
+#endif
+
 #ifndef NDEBUG
     static WTF::String typeAsDebugString(DisplayItem::Type);
 
     void setClientDebugString(const WTF::String& clientDebugString) { m_clientDebugString = clientDebugString; }
     const WTF::String& clientDebugString() const { return m_clientDebugString; }
 
     WTF::String asDebugString() const;
     virtual void dumpPropertiesAsDebugString(WTF::StringBuilder&) const;
 #endif
 
@@ skipping 14 matching lines @@
         }
 
         const DisplayItemClient client;
         const Type type;
     } m_id;
 #ifndef NDEBUG
     WTF::String m_clientDebugString;
 #endif
 };
 
-}
+class PLATFORM_EXPORT PairedBeginDisplayItem : public DisplayItem {
+protected:
+    PairedBeginDisplayItem(DisplayItemClient client, Type type) : DisplayItem(client, type) { }
+
+private:
+    virtual bool isBegin() const override final { return true; }
+};
+
+class PLATFORM_EXPORT PairedEndDisplayItem : public DisplayItem {
+protected:
+    PairedEndDisplayItem(DisplayItemClient client, Type type) : DisplayItem(client, type) { }
+
+#if ENABLE(ASSERT)
+    virtual bool isEndAndPairedWith(const DisplayItem& other) const override = 0;
+#endif
+
+private:
+    virtual bool isEnd() const override final { return true; }
+};
+
+} // namespace blink
 
 #endif // DisplayItem_h