Avoid empty display item pairs

Source/platform/graphics/paint/Transform3DDisplayItem.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 Transform3DDisplayItem_h
 #define Transform3DDisplayItem_h
 
 #include "platform/graphics/paint/DisplayItem.h"
 #include "platform/transforms/TransformationMatrix.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
 
-class PLATFORM_EXPORT BeginTransform3DDisplayItem : public DisplayItem {
+class PLATFORM_EXPORT BeginTransform3DDisplayItem : public PairedBeginDisplayItem {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<BeginTransform3DDisplayItem> create(DisplayItemClient client, const TransformationMatrix& transform)
     {
         return adoptPtr(new BeginTransform3DDisplayItem(client, transform));
     }
 
     BeginTransform3DDisplayItem(DisplayItemClient client, const TransformationMatrix& transform)
-        : DisplayItem(client, BeginTransform)
+        : PairedBeginDisplayItem(client, BeginTransform)
         , m_transform(transform) { }
 
     virtual void replay(GraphicsContext*) override;
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
 private:
     const TransformationMatrix m_transform;
     FloatPoint3D m_transformOrigin;
 };
 
-class PLATFORM_EXPORT EndTransform3DDisplayItem : public DisplayItem {
+class PLATFORM_EXPORT EndTransform3DDisplayItem : public PairedEndDisplayItem {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<EndTransform3DDisplayItem> create(DisplayItemClient client)
     {
         return adoptPtr(new EndTransform3DDisplayItem(client));
     }
 
     EndTransform3DDisplayItem(DisplayItemClient client)
-        : DisplayItem(client, EndTransform) { }
+        : PairedEndDisplayItem(client, EndTransform) { }
 
     virtual void replay(GraphicsContext*) override;
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
+
+private:
+#if ENABLE(ASSERT)
+    // FIXME: Distinguish BeginTransformDisplayItem and BeginTransform3DDisplayItem.
+    virtual bool isEndAndPairedWith(const DisplayItem& other) const override final { return other.type() == BeginTransform; }
+#endif
 };
 
 } // namespace blink
 
 #endif // Transform3DDisplayItem_h