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..93f744632ce01f4a345b5521d1d1c396d4461125 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(yigu): Add detailed comment explaining how
+// StickyPositionScrollingConstraints works.
 class StickyPositionScrollingConstraints final {
  public:
   enum AnchorEdgeFlags {
@@ -113,6 +121,10 @@ class StickyPositionScrollingConstraints final {
     return total_containing_block_sticky_offset_;
   }
 
+  // Returns the relative position of the sticky box and ancestor scroller.
+  FloatSize GetPositionRelativeToScrollAncestor(
+      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