Avoid empty display item pairs

Source/platform/graphics/paint/CompositingDisplayItem.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 CompositingDisplayItem_h
 #define CompositingDisplayItem_h
 
 #include "platform/geometry/LayoutRect.h"
 #include "platform/graphics/GraphicsTypes.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "public/platform/WebBlendMode.h"
 #include "wtf/PassOwnPtr.h"
 #ifndef NDEBUG
 #include "wtf/text/WTFString.h"
 #endif
 
 namespace blink {
 
-class PLATFORM_EXPORT BeginCompositingDisplayItem : public DisplayItem {
+class PLATFORM_EXPORT BeginCompositingDisplayItem : public PairedBeginDisplayItem {
     WTF_MAKE_FAST_ALLOCATED;
 public:
-    static PassOwnPtr<BeginCompositingDisplayItem> create(DisplayItemClient client, Type type, const CompositeOperator preCompositeOp, const WebBlendMode& preBlendMode, const float opacity, const CompositeOperator postCompositeOp)
+    static PassOwnPtr<BeginCompositingDisplayItem> create(DisplayItemClient client, const CompositeOperator preCompositeOp, const WebBlendMode& preBlendMode, const float opacity, const CompositeOperator postCompositeOp)
     {
-        return adoptPtr(new BeginCompositingDisplayItem(client, type, preCompositeOp, preBlendMode, opacity, postCompositeOp));
+        return adoptPtr(new BeginCompositingDisplayItem(client, preCompositeOp, preBlendMode, opacity, postCompositeOp));
     }
 
-    BeginCompositingDisplayItem(DisplayItemClient client, Type type, const CompositeOperator preCompositeOp, const WebBlendMode& preBlendMode, const float opacity, const CompositeOperator postCompositeOp)
-        : DisplayItem(client, type)
+    BeginCompositingDisplayItem(DisplayItemClient client, const CompositeOperator preCompositeOp, const WebBlendMode& preBlendMode, const float opacity, const CompositeOperator postCompositeOp)
+        : PairedBeginDisplayItem(client, BeginCompositing)
         , m_preCompositeOp(preCompositeOp)
         , m_preBlendMode(preBlendMode)
         , m_opacity(opacity)
         , m_postCompositeOp(postCompositeOp) { }
 
     virtual void replay(GraphicsContext*) override;
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
 private:
 #ifndef NDEBUG
     virtual void dumpPropertiesAsDebugString(WTF::StringBuilder&) const override;
 #endif
     const CompositeOperator m_preCompositeOp;
     const WebBlendMode m_preBlendMode;
     const float m_opacity;
     const CompositeOperator m_postCompositeOp;
 };
 
-class PLATFORM_EXPORT EndCompositingDisplayItem : public DisplayItem {
+class PLATFORM_EXPORT EndCompositingDisplayItem : public PairedEndDisplayItem {
     WTF_MAKE_FAST_ALLOCATED;
 public:
-    static PassOwnPtr<EndCompositingDisplayItem> create(DisplayItemClient client, Type type)
+    static PassOwnPtr<EndCompositingDisplayItem> create(DisplayItemClient client)
     {
-        return adoptPtr(new EndCompositingDisplayItem(client, type));
+        return adoptPtr(new EndCompositingDisplayItem(client));
     }
 
-    EndCompositingDisplayItem(DisplayItemClient client, Type type)
-        : DisplayItem(client, type) { }
+    EndCompositingDisplayItem(DisplayItemClient client)
+        : PairedEndDisplayItem(client, EndCompositing) { }
 
     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.type() == BeginCompositing; }
+#endif
 };
 
 } // namespace blink
 
 #endif // CompositingDisplayItem_h