Unify the calculation of main thread offset of sticky element

cc/layers/layer_sticky_position_constraint.h

 // Copyright 2013 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_LAYERS_LAYER_STICKY_POSITION_CONSTRAINT_H_
 #define CC_LAYERS_LAYER_STICKY_POSITION_CONSTRAINT_H_
 
 #include "cc/cc_export.h"
 
 #include "ui/gfx/geometry/point.h"
@@ skipping 11 matching lines @@
   bool is_anchored_top : 1;
   bool is_anchored_bottom : 1;
 
   // The offset from each edge of the ancestor scroller (or the viewport) to
   // try to maintain to the sticky box as we scroll.
   float left_offset;
   float right_offset;
   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
   // the element down from its original position).
   gfx::Rect scroll_container_relative_sticky_box_rect;
 
   // The layout rectangle of the sticky box's containing block relative to the
   // scroll ancestor. The sticky box is only moved as far as its containing
   // block boundary.
   gfx::Rect scroll_container_relative_containing_block_rect;
 
   // The nearest ancestor sticky layer ids that affect the sticky box constraint
   // rect and the containing block constraint rect respectively. If no such
   // layer exists, these are set to Layer::INVALID_ID.
   int nearest_layer_shifting_sticky_box;
   int nearest_layer_shifting_containing_block;
 
   // Returns the nearest sticky ancestor layer, or Layer::INVALID_ID if no such
   // layer exists.
   int NearestStickyAncestor();
 
   bool operator==(const LayerStickyPositionConstraint&) const;
   bool operator!=(const LayerStickyPositionConstraint&) const;
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_STICKY_POSITION_CONSTRAINT_H_