Generate scroll/clip display item hierarchy for SPv2

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 final : public PairedBeginDisplayItem {
 public:
-    BeginFixedPositionDisplayItem(const DisplayItemClientWrapper& client)
-        : PairedBeginDisplayItem(client, BeginFixedPosition, sizeof(*this)) { }
+    BeginFixedPositionDisplayItem(const DisplayItemClientWrapper& client, const DisplayItemClientWrapper& anchor)
+        : PairedBeginDisplayItem(client, BeginFixedPosition, sizeof(*this)), m_anchor(anchor.displayItemClient()) { }
 
     void replay(GraphicsContext&) final { }
     void appendToWebDisplayItemList(WebDisplayItemList*) const final;
+
+    DisplayItemClient anchor() const { return m_anchor; }
+
+    // Note: This is only temporarily workaround before we implement true
+    // transform-tree-based rasterization. Do not use.
+    void setCounterScroll(const FloatSize& counterScroll) const { m_counterScroll = counterScroll; }
+
+private:
+    // Note: This is only temporarily workaround before we implement true
+    // transform-tree-based rasterization. Do not use.
+    mutable FloatSize m_counterScroll;
+
+    DisplayItemClient m_anchor;
 };
 
 class PLATFORM_EXPORT EndFixedPositionDisplayItem final : public PairedEndDisplayItem {
 public:
     EndFixedPositionDisplayItem(const DisplayItemClientWrapper& client)
         : PairedEndDisplayItem(client, EndFixedPosition, sizeof(*this)) { }
 
     void replay(GraphicsContext&) final { }
     void appendToWebDisplayItemList(WebDisplayItemList*) const final;
 
 private:
 #if ENABLE(ASSERT)
     bool isEndAndPairedWith(DisplayItem::Type otherType) const final { return otherType == BeginFixedPosition; }
 #endif
 };
 
 } // namespace blink
 
 #endif // FixedPositionDisplayItem_h