Adapt and reland old position sticky implementation

Source/core/layout/LayoutGeometryMapStep.h

Index: Source/core/layout/LayoutGeometryMapStep.h
diff --git a/Source/core/layout/LayoutGeometryMapStep.h b/Source/core/layout/LayoutGeometryMapStep.h
index 4613d3747005def98f93b02291e4e0407ac9e2d6..b346335a796a1ce7294a02885113a8023db1d07f 100644
--- a/Source/core/layout/LayoutGeometryMapStep.h
+++ b/Source/core/layout/LayoutGeometryMapStep.h
@@ -42,29 +42,29 @@ struct LayoutGeometryMapStep {
     LayoutGeometryMapStep(const LayoutGeometryMapStep& o)
         : m_layoutObject(o.m_layoutObject)
         , m_offset(o.m_offset)
-        , m_offsetForFixedPosition(o.m_offsetForFixedPosition)
+        , m_offsetForViewportConstrained(o.m_offsetForViewportConstrained)
         , m_accumulatingTransform(o.m_accumulatingTransform)
         , m_isNonUniform(o.m_isNonUniform)
-        , m_isFixedPosition(o.m_isFixedPosition)
+        , m_isViewportConstrained(o.m_isViewportConstrained)
         , m_hasTransform(o.m_hasTransform)
     {
         ASSERT(!o.m_transform);
     }
-    LayoutGeometryMapStep(const LayoutObject* layoutObject, bool accumulatingTransform, bool isNonUniform, bool isFixedPosition, bool hasTransform)
+    LayoutGeometryMapStep(const LayoutObject* layoutObject, bool accumulatingTransform, bool isNonUniform, bool isViewportConstrained, bool hasTransform)
         : m_layoutObject(layoutObject)
         , m_accumulatingTransform(accumulatingTransform)
         , m_isNonUniform(isNonUniform)
-        , m_isFixedPosition(isFixedPosition)
+        , m_isViewportConstrained(isViewportConstrained)
         , m_hasTransform(hasTransform)
     {
     }
     const LayoutObject* m_layoutObject;
     LayoutSize m_offset;
     OwnPtr<TransformationMatrix> m_transform; // Includes offset if non-null.
-    LayoutSize m_offsetForFixedPosition;
+    LayoutSize m_offsetForViewportConstrained;
     bool m_accumulatingTransform;
     bool m_isNonUniform; // Mapping depends on the input point, e.g. because of CSS columns.
-    bool m_isFixedPosition;
+    bool m_isViewportConstrained;
     bool m_hasTransform;
 };