Add map from element id to scroll node index.

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index 653189d0c44f8023c501883be9a16b3d641f03de..711c8ce91bbd992b1a4a27b551a16820999109cb 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -544,6 +544,8 @@ class CC_EXPORT PropertyTrees final {
   std::unordered_map<ElementId, int, ElementIdHash>
       element_id_to_effect_node_index;
   std::unordered_map<ElementId, int, ElementIdHash>
+      element_id_to_scroll_node_index;
+  std::unordered_map<ElementId, int, ElementIdHash>
       element_id_to_transform_node_index;
 
   std::vector<int> always_use_active_tree_opacity_effect_ids;