Avoid empty display item pairs

Source/platform/graphics/paint/ClipPathDisplayItem.h

 // Copyright 2015 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 ClipPathDisplayItem_h
 #define ClipPathDisplayItem_h
 
 #include "platform/PlatformExport.h"
 #include "platform/graphics/Path.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
 
-class PLATFORM_EXPORT BeginClipPathDisplayItem : public DisplayItem {
+class PLATFORM_EXPORT BeginClipPathDisplayItem : public PairedBeginDisplayItem {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<BeginClipPathDisplayItem> create(DisplayItemClient client, const Path& clipPath, WindRule windRule)
     {
         return adoptPtr(new BeginClipPathDisplayItem(client, clipPath, windRule));
     }
 
     BeginClipPathDisplayItem(DisplayItemClient client, const Path& clipPath, WindRule windRule)
-        : DisplayItem(client, BeginClipPath)
+        : PairedBeginDisplayItem(client, BeginClipPath)
         , m_clipPath(clipPath)
         , m_windRule(windRule) { }
 
     virtual void replay(GraphicsContext*) override;
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
 private:
     const Path m_clipPath;
     const WindRule m_windRule;
 #ifndef NDEBUG
     virtual void dumpPropertiesAsDebugString(WTF::StringBuilder&) const override;
 #endif
 };
 
-class PLATFORM_EXPORT EndClipPathDisplayItem : public DisplayItem {
+class PLATFORM_EXPORT EndClipPathDisplayItem : public PairedEndDisplayItem {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<EndClipPathDisplayItem> create(DisplayItemClient client)
     {
         return adoptPtr(new EndClipPathDisplayItem(client));
     }
 
     EndClipPathDisplayItem(DisplayItemClient client)
-        : DisplayItem(client, EndClipPath) { }
+        : PairedEndDisplayItem(client, EndClipPath) { }
 
     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() == BeginClipPath; }
+#endif
 };
 
 } // namespace blink
 
 #endif // ClipPathDisplayItem_h