Unify the calculation of main thread offset of sticky element

third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h

Index: third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
diff --git a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
index 330d2d86334c79ac8050a50cc8565bc97c06bfeb..428531757cec98cfd1175c4089b07cf026a6d1cb 100644
--- a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
+++ b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
@@ -7,11 +7,19 @@
 
 #include "platform/geometry/FloatRect.h"
 #include "platform/geometry/FloatSize.h"
+#include "platform/wtf/HashMap.h"
 
 namespace blink {
 
 class LayoutBoxModelObject;
+class PaintLayer;
+class StickyPositionScrollingConstraints;
 
+typedef WTF::HashMap<PaintLayer*, StickyPositionScrollingConstraints>
+    StickyConstraintsMap;
+
+// TODO(smcgruer): Add detailed comment explaining how
+// StickyPositionScrollingConstraints works.
 class StickyPositionScrollingConstraints final {
  public:
   enum AnchorEdgeFlags {
@@ -113,6 +121,11 @@ class StickyPositionScrollingConstraints final {
     return total_containing_block_sticky_offset_;
   }
 
+  // Returns the relative position of the sticky box and its original position
+  // before scroll. This method is only safe to call if ComputeStickyOffset has
+  // been invoked.
+  FloatSize GetOffsetForStickyPosition(const StickyConstraintsMap&) const;
+
   const LayoutBoxModelObject* NearestStickyAncestor() const {
     // If we have one or more sticky ancestor elements between ourselves and our
     // containing block, |m_nearestStickyBoxShiftingStickyBox| points to the