Unify the calculation of main thread offset of sticky element

cc/trees/property_tree.cc

Index: cc/trees/property_tree.cc
diff --git a/cc/trees/property_tree.cc b/cc/trees/property_tree.cc
index a64099b265b3723cf97ea2380650c6135193b0dd..89445a11614dd5ab436c31da70fd39eeeb3ed227 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -378,7 +378,6 @@ gfx::Vector2dF StickyPositionOffset(TransformTree* tree, TransformNode* node) {
       scroll_position,
       gfx::SizeF(property_trees.scroll_tree.scroll_clip_layer_bounds(
           scroll_node->id)));
-  gfx::Vector2dF layer_offset(sticky_data->main_thread_offset);
 
   gfx::Vector2dF ancestor_sticky_box_offset;
   if (sticky_data->nearest_node_shifting_sticky_box !=
@@ -463,7 +462,7 @@ gfx::Vector2dF StickyPositionOffset(TransformTree* tree, TransformNode* node) {
       ancestor_sticky_box_offset + ancestor_containing_block_offset +
       sticky_offset - sticky_box_rect.OffsetFromOrigin();
 
-  return sticky_offset - layer_offset - node->source_to_parent -
+  return sticky_offset - node->sticky_main_thread_offset -
          sticky_box_rect.OffsetFromOrigin();
 }