Unify the calculation of main thread offset of sticky element

cc/trees/property_tree.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_PROPERTY_TREE_H_
 #define CC_TREES_PROPERTY_TREE_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 310 matching lines @@
   std::vector<int> nodes_affected_by_inner_viewport_bounds_delta_;
   std::vector<int> nodes_affected_by_outer_viewport_bounds_delta_;
   std::vector<TransformCachedNodeData> cached_data_;
   std::vector<StickyPositionNodeData> sticky_position_data_;
 };
 
 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
   // used to correct the constraining rect locations.
   int nearest_node_shifting_sticky_box;
   int nearest_node_shifting_containing_block;
 
   // For performance we cache our accumulated sticky offset to allow descendant
   // sticky elements to offset their constraint rects. Because we can either
   // affect the sticky box constraint rect or the containing block constraint
@@ skipping 409 matching lines @@
   DrawTransforms& GetDrawTransforms(int transform_id, int effect_id) const;
   DrawTransformData& FetchDrawTransformsDataFromCache(int transform_id,
                                                       int effect_id) const;
 
   PropertyTreesCachedData cached_data_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_PROPERTY_TREE_H_