Avoid empty display item pairs

Source/platform/graphics/paint/CompositingDisplayItem.h

Index: Source/platform/graphics/paint/CompositingDisplayItem.h
diff --git a/Source/platform/graphics/paint/CompositingDisplayItem.h b/Source/platform/graphics/paint/CompositingDisplayItem.h
index 65643391499ef97d08b698104628e9b109de94dd..51d24f6f259e4d6608afb844d93fe7330b3625cc 100644
--- a/Source/platform/graphics/paint/CompositingDisplayItem.h
+++ b/Source/platform/graphics/paint/CompositingDisplayItem.h
@@ -16,16 +16,16 @@
 
 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)
@@ -44,19 +44,24 @@ private:
     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