Avoid empty display item pairs

Source/platform/graphics/paint/FloatClipDisplayItem.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 FloatClipDisplayItem_h
 #define FloatClipDisplayItem_h
 
 #include "platform/PlatformExport.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
 
 class RoundedRect;
 
-class PLATFORM_EXPORT FloatClipDisplayItem : public DisplayItem {
+class PLATFORM_EXPORT FloatClipDisplayItem : public PairedBeginDisplayItem {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<FloatClipDisplayItem> create(DisplayItemClient client, Type type, const FloatRect& clipRect)
     {
         return adoptPtr(new FloatClipDisplayItem(client, type, clipRect));
     }
 
     FloatClipDisplayItem(DisplayItemClient client, Type type, const FloatRect& clipRect)
-        : DisplayItem(client, type)
+        : PairedBeginDisplayItem(client, type)
         , m_clipRect(clipRect)
     {
         ASSERT(isFloatClipType(type));
     }
 
     virtual void replay(GraphicsContext*) override;
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
 private:
 #ifndef NDEBUG
     virtual void dumpPropertiesAsDebugString(WTF::StringBuilder&) const override;
 #endif
 
     FloatRect m_clipRect;
 };
 
-class PLATFORM_EXPORT EndFloatClipDisplayItem : public DisplayItem {
+class PLATFORM_EXPORT EndFloatClipDisplayItem : public PairedEndDisplayItem {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<EndFloatClipDisplayItem> create(DisplayItemClient client, Type type)
     {
         return adoptPtr(new EndFloatClipDisplayItem(client, type));
     }
 
     EndFloatClipDisplayItem(DisplayItemClient client, Type type)
-        : DisplayItem(client, type)
+        : PairedEndDisplayItem(client, type)
     {
         ASSERT(isEndFloatClipType(type));
     }
 
     virtual void replay(GraphicsContext*) override;
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
+
+private:
+#if ENABLE(ASSERT)
+    virtual bool isEndAndPairedWith(const DisplayItem& other) const override final { return other.isFloatClip(); }
+#endif
 };
 
 } // namespace blink
 
 #endif // FloatClipDisplayItem_h