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 f450baa7067a5b3aa83fa9b6f08a201f17eb2e24..7fe6f360a987c7d3a26a8948605c76df543d5820 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -372,7 +372,6 @@ 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 !=
@@ -457,7 +456,11 @@ 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 -
+  gfx::Vector2dF sticky_main_thread_offset(

[flackr, 2017/05/31 14:08:22]

Rather than converting to a Vector2dF every time, perhaps we should store this
as a Vector2dF. This also matches many of the other offsets on TransformNode.

[yigu, 2017/06/01 15:15:19]

Done.

+      node->sticky_main_thread_offset.width(),
+      node->sticky_main_thread_offset.height());
+
+  return sticky_offset - sticky_main_thread_offset -
          sticky_box_rect.OffsetFromOrigin();
 }