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..205c65c9fa6984a5d196d907cc99ef065a15ff77 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 +121,11 @@ class StickyPositionScrollingConstraints final {
     return total_containing_block_sticky_offset_;
   }
 
+  // Returns the relative position of the sticky box and ancestor scroller so
+  // that it can figure out how much the main thread has already pushed this

[chrishtr, 2017/06/05 18:28:45]

You shouldn't specify use cases. It's clearer to just say what this
represents.

[yigu, 2017/06/05 18:35:08]

Done.

+  // sticky element down from setting the layer position.
+  FloatSize GetLocalStickyOffset(const StickyConstraintsMap&) const;

[chrishtr, 2017/06/05 18:28:45]

GetPositionRelativeToScrollAncestor?

[yigu, 2017/06/05 18:35:08]

Done.

+
   const LayoutBoxModelObject* NearestStickyAncestor() const {
     // If we have one or more sticky ancestor elements between ourselves and our
     // containing block, |m_nearestStickyBoxShiftingStickyBox| points to the