Unify the calculation of main thread offset of sticky element

cc/trees/transform_node.h

 // Copyright 2016 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_TRANSFORM_NODE_H_
 #define CC_TREES_TRANSFORM_NODE_H_
 
 #include "cc/cc_export.h"
 #include "ui/gfx/geometry/point_f.h"
 #include "ui/gfx/geometry/scroll_offset.h"
@@ skipping 37 matching lines @@
   gfx::Transform pre_local;
   gfx::Transform local;
   gfx::Transform post_local;
 
   gfx::Transform to_parent;
 
   // This is the node which defines the sticky position constraints for this
   // transform node. -1 indicates there are no sticky position constraints.
   int sticky_position_constraint_id;
 
+  // This stores the main thread offset applied to the sticky element from main
+  // thread. It's used to calculate the actual sticky position offset from cc.

[flackr, 2017/06/01 19:01:47]

nit: s/actual sticky position offset/additional sticky position offset

[yigu, 2017/06/01 19:28:40]

Done.

+  gfx::Vector2dF sticky_main_thread_offset;
+
   // This is the node with respect to which source_offset is defined. This will
   // not be needed once layerization moves to cc, but is needed in order to
   // efficiently update the transform tree for changes to position in the layer
   // tree.
   int source_node_id;
 
   // This id determines which 3d rendering context the node is in. 0 is a
   // special value and indicates that the node is not in any 3d rendering
   // context.
   int sorting_context_id;
@@ skipping 95 matching lines @@
   gfx::Transform to_screen;
 
   bool is_showing_backface : 1;
 
   bool operator==(const TransformCachedNodeData& other) const;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_TRANSFORM_NODE_H_