Generate scroll/clip display item hierarchy for SPv2

Source/platform/graphics/paint/FixedPositionDisplayItem.h

Index: Source/platform/graphics/paint/FixedPositionDisplayItem.h
diff --git a/Source/platform/graphics/paint/FixedPositionDisplayItem.h b/Source/platform/graphics/paint/FixedPositionDisplayItem.h
index ce2bf6dc063d297ff21b319eac09ef89a52ad5b8..278f57d588030919c7dc1139708f49752b0237f0 100644
--- a/Source/platform/graphics/paint/FixedPositionDisplayItem.h
+++ b/Source/platform/graphics/paint/FixedPositionDisplayItem.h
@@ -14,11 +14,24 @@ 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 {