Unify the calculation of main thread offset of sticky element

cc/trees/transform_node.h

Index: cc/trees/transform_node.h
diff --git a/cc/trees/transform_node.h b/cc/trees/transform_node.h
index 2abfb1cc00480b7eb14ed6dab868d15a165a4096..6ecff1cad106cf82429d3fdc3158958678fc301c 100644
--- a/cc/trees/transform_node.h
+++ b/cc/trees/transform_node.h
@@ -55,6 +55,8 @@ struct CC_EXPORT TransformNode {
   // transform node. -1 indicates there are no sticky position constraints.
   int sticky_position_constraint_id;
 
+  gfx::Size 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