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..8da5815be353549715c06d17a71eedaaed59ffb1 100644
--- a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
+++ b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
@@ -7,10 +7,16 @@
 
 #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;
 
 class StickyPositionScrollingConstraints final {
  public:
@@ -113,6 +119,8 @@ class StickyPositionScrollingConstraints final {
     return total_containing_block_sticky_offset_;
   }
 
+  FloatSize GetLocalStickyOffset(const StickyConstraintsMap&) const;

[chrishtr, 2017/06/05 17:17:34]

Document clearly what this returns.

Not your fault, but: actually, on further inspection
of this file, each of these public accessors and setters needs documentation.
I suggest a class-level comment explaining each of the different types of
information, with examples.

[yigu, 2017/06/05 18:16:34]

Added a TODO for adding the class-level comment with examples.

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