Reland main thread position sticky implementation.

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

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 33 matching lines @@
 };
 typedef unsigned GeometryInfoFlags;
 
 // Stores data about how to map from one layoutObject to its container.
 struct LayoutGeometryMapStep {
     DISALLOW_NEW_EXCEPT_PLACEMENT_NEW();
     LayoutGeometryMapStep(const LayoutGeometryMapStep& o)
         : m_layoutObject(o.m_layoutObject)
         , m_offset(o.m_offset)
         , m_offsetForFixedPosition(o.m_offsetForFixedPosition)
+        , m_offsetForStickyPosition(o.m_offsetForStickyPosition)
         , m_flags(o.m_flags)
     {
         ASSERT(!o.m_transform);
     }
     LayoutGeometryMapStep(const LayoutObject* layoutObject, GeometryInfoFlags flags)
         : m_layoutObject(layoutObject)
         , m_flags(flags)
     {
     }
     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;
     GeometryInfoFlags m_flags;
 };
 
 } // namespace blink
 
 WTF_ALLOW_MOVE_INIT_AND_COMPARE_WITH_MEM_FUNCTIONS(blink::LayoutGeometryMapStep);
 
 #endif // LayoutGeometryMapStep_h