[Reland] Correct fixed-position recording for Slimming Paint

Source/platform/graphics/paint/FixedPositionDisplayItem.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 FixedPositionDisplayItem_h
+#define FixedPositionDisplayItem_h
+
+#include "platform/geometry/LayoutSize.h"
+#include "platform/graphics/paint/DisplayItem.h"
+#include "wtf/FastAllocBase.h"
+#include "wtf/PassOwnPtr.h"
+
+namespace blink {
+
+class PLATFORM_EXPORT BeginFixedPositionDisplayItem : public PairedBeginDisplayItem {
+    WTF_MAKE_FAST_ALLOCATED(BeginFixedPositionDisplayItem);
+public:
+    static PassOwnPtr<BeginFixedPositionDisplayItem> create(const DisplayItemClientWrapper& client)
+    {
+        return adoptPtr(new BeginFixedPositionDisplayItem(client));
+    }
+
+    BeginFixedPositionDisplayItem(const DisplayItemClientWrapper& client)
+        : PairedBeginDisplayItem(client, BeginFixedPosition)
+    {
+    }
+
+    virtual void replay(GraphicsContext&) override final { }
+    virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override final;
+};
+
+class PLATFORM_EXPORT EndFixedPositionDisplayItem : public PairedEndDisplayItem {
+    WTF_MAKE_FAST_ALLOCATED(EndFixedPositionDisplayItem);
+public:
+    static PassOwnPtr<EndFixedPositionDisplayItem> create(const DisplayItemClientWrapper& client)
+    {
+        return adoptPtr(new EndFixedPositionDisplayItem(client));
+    }
+
+    EndFixedPositionDisplayItem(const DisplayItemClientWrapper& client)
+        : PairedEndDisplayItem(client, EndFixedPosition)
+    {
+    }
+
+    virtual void replay(GraphicsContext&) override final { }
+    virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override final;
+
+private:
+#if ENABLE(ASSERT)
+    virtual bool isEndAndPairedWith(const DisplayItem& other) const override final { return other.type() == BeginFixedPosition; }
+#endif
+};
+
+} // namespace blink
+
+#endif // FixedPositionDisplayItem_h