Unify the calculation of main thread offset of sticky element

third_party/WebKit/public/platform/WebLayerStickyPositionConstraint.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 33 matching lines @@
   bool is_anchored_top : 1;
   bool is_anchored_bottom : 1;
 
   // For each edge, when the anchored bit above is set this gives the distance
   // to keep the element from that edge of the ancestor scroller or viewport.
   float left_offset;
   float right_offset;
   float top_offset;
   float bottom_offset;
 
-  // This is the layout position of the sticky element before it has been
-  // shifted relative to the enclosing composited layer.
-  WebPoint parent_relative_sticky_box_offset;
-
   // The layout rectangle of the sticky element before it has been shifted
   // to stick.
   WebRect scroll_container_relative_sticky_box_rect;
 
   // The layout rectangle of the containing block edges which this sticky
   // element should not be shifted beyond.
   WebRect scroll_container_relative_containing_block_rect;
 
   // The nearest ancestor sticky layers that affect the sticky box constraint
   // rect and the containing block constraint rect respectively.
@@ skipping 10 matching lines @@
         right_offset(0.f),
         top_offset(0.f),
         bottom_offset(0.f),
         nearest_layer_shifting_sticky_box(WebLayer::kInvalidLayerId),
         nearest_layer_shifting_containing_block(WebLayer::kInvalidLayerId) {}
 };
 
 }  // namespace blink
 
 #endif  // WebLayerStickyPositionConstraint_h