Implement position: sticky updates on compositor

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 7893be2e4e0d871c501e37e5e539c7c96d0f91c4..1e8c3910aaa08a8449e3a5136995e0f6661e04eb 100644
--- a/cc/trees/transform_node.h
+++ b/cc/trees/transform_node.h
@@ -53,6 +53,10 @@ struct CC_EXPORT TransformNode {
 
   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 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