Add map from element id to scroll node index.

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 31a216a9da9604bbd81d8a71715efcda4ecec815..ef0a4e3ed160df3ddee4cccdf0f1bb7d4ea305c0 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -1538,6 +1538,8 @@ bool PropertyTrees::operator==(const PropertyTrees& other) const {
          scroll_id_to_index_map == other.scroll_id_to_index_map &&
          element_id_to_effect_node_index ==
              other.element_id_to_effect_node_index &&
+         element_id_to_scroll_node_index ==
+             other.element_id_to_scroll_node_index &&
          element_id_to_transform_node_index ==
              other.element_id_to_transform_node_index &&
          always_use_active_tree_opacity_effect_ids ==
@@ -1562,6 +1564,7 @@ PropertyTrees& PropertyTrees::operator=(const PropertyTrees& from) {
   clip_id_to_index_map = from.clip_id_to_index_map;
   scroll_id_to_index_map = from.scroll_id_to_index_map;
   element_id_to_effect_node_index = from.element_id_to_effect_node_index;
+  element_id_to_scroll_node_index = from.element_id_to_scroll_node_index;
   element_id_to_transform_node_index = from.element_id_to_transform_node_index;
   needs_rebuild = from.needs_rebuild;
   changed = from.changed;
@@ -1594,6 +1597,7 @@ void PropertyTrees::clear() {
   clip_id_to_index_map.clear();
   scroll_id_to_index_map.clear();
   element_id_to_effect_node_index.clear();
+  element_id_to_scroll_node_index.clear();
   element_id_to_transform_node_index.clear();
   always_use_active_tree_opacity_effect_ids.clear();