Unify the calculation of main thread offset of sticky element

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index 08d4dcd4e38fb9b91e396402fef9c2a2e4a08ae7..47fbc5a22c414312ff7a649afdb3e4bbc8fd4c97 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -311,11 +311,6 @@ struct StickyPositionNodeData {
   int scroll_ancestor;
   LayerStickyPositionConstraint constraints;
 
-  // This is the offset that blink has already applied to counteract the main
-  // thread scroll offset of the scroll ancestor. We need to account for this
-  // by computing the additional offset necessary to keep the element stuck.
-  gfx::Vector2dF main_thread_offset;
-
   // In order to properly compute the sticky offset, we need to know if we have
   // any sticky ancestors both between ourselves and our containing block and
   // between our containing block and the viewport. These ancestors are then