Shifting layer position for sticky element to avoid passing unnessary variable to cc

cc/trees/property_tree.cc

 // 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.
 
 #include <stddef.h>
 
 #include <set>
 #include <vector>
 
 #include "base/logging.h"
@@ skipping 335 matching lines @@
   // from the destination to the source, with flattening, and then invert the
   // result.
   gfx::Transform dest_to_source;
   CombineTransformsBetween(dest_id, source_id, &dest_to_source);
   gfx::Transform source_to_dest;
   bool all_are_invertible = dest_to_source.GetInverse(&source_to_dest);
   transform->PreconcatTransform(source_to_dest);
   return all_are_invertible;
 }
 
 gfx::Vector2dF StickyPositionOffset(TransformTree* tree, TransformNode* node) {

[flackr, 2017/06/08 18:56:18]

I think now it definitely makes sense to add the comment that trchen mentioned
in your last review:
https://codereview.chromium.org/2911463002/diff/200001/cc/trees/property_tree...

These functions need to be kept in sync (in case changes are made in one)

[yigu, 2017/06/08 23:30:03]

Done.

   if (node->sticky_position_constraint_id == -1)
     return gfx::Vector2dF();
   StickyPositionNodeData* sticky_data = tree->StickyPositionData(node->id);
   const LayerStickyPositionConstraint& constraint = sticky_data->constraints;
   auto& property_trees = *tree->property_trees();
   ScrollNode* scroll_node =
       property_trees.scroll_tree.Node(sticky_data->scroll_ancestor);
   TransformNode* transform_node =
       property_trees.transform_tree.Node(scroll_node->transform_id);
   const auto& scroll_offset = transform_node->scroll_offset;
@@ skipping 87 matching lines @@
       top_delta = available_space;
     sticky_offset.set_y(sticky_offset.y() + top_delta);
   }
 
   sticky_data->total_sticky_box_sticky_offset =
       ancestor_sticky_box_offset + sticky_offset;
   sticky_data->total_containing_block_sticky_offset =
       ancestor_sticky_box_offset + ancestor_containing_block_offset +
       sticky_offset;
 
-  return sticky_offset - node->offset_for_sticky_position_from_main_thread;
+  return sticky_offset;
 }
 
 void TransformTree::UpdateLocalTransform(TransformNode* node) {
   gfx::Transform transform = node->post_local;
   if (NeedsSourceToParentUpdate(node)) {
     gfx::Transform to_parent;
     ComputeTranslation(node->source_node_id, node->parent_id, &to_parent);
     gfx::Vector2dF unsnapping;
     TransformNode* current;
     TransformNode* parent_node;
@@ skipping 1718 matching lines @@
   const EffectNode* effect_node = effect_tree.Node(effect_id);
 
   if (effect_node->surface_contents_scale.x() != 0.0 &&
       effect_node->surface_contents_scale.y() != 0.0)
     screen_space_transform.Scale(1.0 / effect_node->surface_contents_scale.x(),
                                  1.0 / effect_node->surface_contents_scale.y());
   return screen_space_transform;
 }
 
 }  // namespace cc