Avoid empty display item pairs

Source/platform/graphics/paint/ClipDisplayItem.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 ClipDisplayItem_h
 #define ClipDisplayItem_h
 
 #include "SkRegion.h"
 #include "platform/PlatformExport.h"
 #include "platform/geometry/FloatRoundedRect.h"
 #include "platform/geometry/IntRect.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/Vector.h"
 
 namespace blink {
 
-class PLATFORM_EXPORT ClipDisplayItem : public DisplayItem {
+class PLATFORM_EXPORT ClipDisplayItem : public PairedBeginDisplayItem {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<ClipDisplayItem> create(DisplayItemClient client, Type type, const IntRect& clipRect, SkRegion::Op operation = SkRegion::kIntersect_Op)
     {
         return adoptPtr(new ClipDisplayItem(client, type, clipRect, operation));
     }
 
     ClipDisplayItem(DisplayItemClient client, Type type, const IntRect& clipRect, SkRegion::Op operation = SkRegion::kIntersect_Op)
-        : DisplayItem(client, type)
+        : PairedBeginDisplayItem(client, type)
         , m_clipRect(clipRect)
         , m_operation(operation)
     {
         ASSERT(isClipType(type));
     }
 
     virtual void replay(GraphicsContext*) override;
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
     Vector<FloatRoundedRect>& roundedRectClips() { return m_roundedRectClips; }
 
 private:
 #ifndef NDEBUG
     virtual void dumpPropertiesAsDebugString(WTF::StringBuilder&) const override;
 #endif
     IntRect m_clipRect;
     Vector<FloatRoundedRect> m_roundedRectClips;
     SkRegion::Op m_operation;
 };
 
-class PLATFORM_EXPORT EndClipDisplayItem : public DisplayItem {
+class PLATFORM_EXPORT EndClipDisplayItem : public PairedEndDisplayItem {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<EndClipDisplayItem> create(DisplayItemClient client, Type type)
     {
         return adoptPtr(new EndClipDisplayItem(client, type));
     }
 
     EndClipDisplayItem(DisplayItemClient client, Type type)
-        : DisplayItem(client, type)
+        : PairedEndDisplayItem(client, type)
     {
-        isEndClipType(type);
+        ASSERT(isEndClipType(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.isClip(); }
+#endif
 };
 
 } // namespace blink
 
 #endif // ClipDisplayItem_h