Unify the calculation of main thread offset of sticky element

cc/layers/layer_sticky_position_constraint.h

Index: cc/layers/layer_sticky_position_constraint.h
diff --git a/cc/layers/layer_sticky_position_constraint.h b/cc/layers/layer_sticky_position_constraint.h
index adf053528f541aa570e23dd4ee477f006500e9be..782fa9426f6390c9972fb95bcd47456859e6f018 100644
--- a/cc/layers/layer_sticky_position_constraint.h
+++ b/cc/layers/layer_sticky_position_constraint.h
@@ -29,11 +29,6 @@ struct CC_EXPORT LayerStickyPositionConstraint {
   float top_offset;
   float bottom_offset;
 
-  // The layout offset of the sticky box relative to its containing layer.
-  // This is used to detect the sticky offset the main thread has applied
-  // to the layer.
-  gfx::Point parent_relative_sticky_box_offset;
-
   // The rectangle corresponding to original layout position of the sticky box
   // relative to the scroll ancestor. The sticky box is only offset once the
   // scroll has passed its initial position (e.g. top_offset will only push