Adapt and reland old position sticky implementation

third_party/WebKit/Source/core/layout/LayoutGeometryMapStep.h

Index: third_party/WebKit/Source/core/layout/LayoutGeometryMapStep.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutGeometryMapStep.h b/third_party/WebKit/Source/core/layout/LayoutGeometryMapStep.h
index ad70e683ada09bdbd50bb3f5aa45542aa102cbdd..1cfc4df29190ed923eebb9a9774c5b03bf91da6d 100644
--- a/third_party/WebKit/Source/core/layout/LayoutGeometryMapStep.h
+++ b/third_party/WebKit/Source/core/layout/LayoutGeometryMapStep.h
@@ -43,6 +43,7 @@ struct LayoutGeometryMapStep {
         : m_layoutObject(o.m_layoutObject)
         , m_offset(o.m_offset)
         , m_offsetForFixedPosition(o.m_offsetForFixedPosition)
+        , m_offsetForStickyPosition(o.m_offsetForStickyPosition)
         , m_accumulatingTransform(o.m_accumulatingTransform)
         , m_isNonUniform(o.m_isNonUniform)
         , m_isFixedPosition(o.m_isFixedPosition)
@@ -61,7 +62,10 @@ struct LayoutGeometryMapStep {
     const LayoutObject* m_layoutObject;
     LayoutSize m_offset;
     OwnPtr<TransformationMatrix> m_transform; // Includes offset if non-null.
+    // If m_offsetForFixedPosition could only apply to the fixed position steps, we may be able to merge
+    // with m_offsetForStickyPosition and simplify mapping.
     LayoutSize m_offsetForFixedPosition;
+    LayoutSize m_offsetForStickyPosition;
     bool m_accumulatingTransform;
     bool m_isNonUniform; // Mapping depends on the input point, e.g. because of CSS columns.
     bool m_isFixedPosition;