Unify the calculation of main thread offset of sticky element

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

Index: third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
diff --git a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
index f099cdce40417819ed11a11f133cdf54b503a75f..61150ecbbc4485b164011349d4eae3a78b547eee 100644
--- a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "core/page/scrolling/StickyPositionScrollingConstraints.h"
+#include "core/paint/PaintLayer.h"
 
 namespace blink {
 
@@ -88,4 +89,16 @@ FloatSize StickyPositionScrollingConstraints::ComputeStickyOffset(
   return sticky_offset;
 }
 
+FloatSize StickyPositionScrollingConstraints::GetOffsetForStickyPosition(
+    const StickyConstraintsMap& constraints_map) const {
+  FloatSize nearest_sticky_box_shifting_sticky_box_constraints_offset;
+  if (nearest_sticky_box_shifting_sticky_box_) {
+    nearest_sticky_box_shifting_sticky_box_constraints_offset =
+        constraints_map.at(nearest_sticky_box_shifting_sticky_box_->Layer())
+            .GetTotalStickyBoxStickyOffset();
+  }
+  return total_sticky_box_sticky_offset_ -
+         nearest_sticky_box_shifting_sticky_box_constraints_offset;
+}
+
 }  // namespace blink