Unify the calculation of main thread offset of sticky element

cc/trees/property_tree.cc

Index: cc/trees/property_tree.cc
diff --git a/cc/trees/property_tree.cc b/cc/trees/property_tree.cc
index a64099b265b3723cf97ea2380650c6135193b0dd..89445a11614dd5ab436c31da70fd39eeeb3ed227 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -378,7 +378,6 @@ gfx::Vector2dF StickyPositionOffset(TransformTree* tree, TransformNode* node) {
       scroll_position,
       gfx::SizeF(property_trees.scroll_tree.scroll_clip_layer_bounds(
           scroll_node->id)));
-  gfx::Vector2dF layer_offset(sticky_data->main_thread_offset);
 
   gfx::Vector2dF ancestor_sticky_box_offset;
   if (sticky_data->nearest_node_shifting_sticky_box !=
@@ -463,7 +462,7 @@ gfx::Vector2dF StickyPositionOffset(TransformTree* tree, TransformNode* node) {
       ancestor_sticky_box_offset + ancestor_containing_block_offset +
       sticky_offset - sticky_box_rect.OffsetFromOrigin();
 
-  return sticky_offset - layer_offset - node->source_to_parent -
+  return sticky_offset - node->sticky_main_thread_offset -
          sticky_box_rect.OffsetFromOrigin();

[trchen, 2017/06/05 22:43:11]

This is quite confusing. I think the logic is to compute the local sticky offset
using only layout information (prior to blink adjustment), then subtract blink
adjustment from target adjustment, i.e. 

(sticky_offset - sticky_box_rect.OffsetFromOrigin()) -
node->sticky_main_thread_offset

which is the same value, but with correct associating order.

However it raised the question why sticky_offset included
sticky_box_rect.OffsetFromOrigin() in the first place. Can we just do

gfx::Vector2dF sticky_offset;

on line #408? Then we can replace "sticky_offset -
sticky_box_rect.OffsetFromOrigin()" with just "sticky_offset" on line #458 #461
#465.

[yigu, 2017/06/06 19:33:18]

Done.

 }