Add map from element id to scroll node index.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index f4ce94d2bd4be5f8189366801b9704f29dc308c9..0b89de5202f084738f77955bcc885f4a53990da2 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -3941,16 +3941,14 @@ void LayerTreeHostImpl::SetTreeLayerScrollOffsetMutated(
       1u, property_trees->element_id_to_transform_node_index.count(element_id));
   int transform_node_index =
       property_trees->element_id_to_transform_node_index[element_id];
-  // TODO(wkorman): Build map from element id to scroll node in property tree
-  // builder and make use of it below.
+  DCHECK_EQ(1u,
+            property_trees->element_id_to_scroll_node_index.count(element_id));
   const int scroll_node_index =
-      property_trees->layer_id_to_scroll_node_index[layer_id];
-  if (scroll_node_index != ScrollTree::kInvalidNodeId) {
-    property_trees->scroll_tree.OnScrollOffsetAnimated(
-        layer_id, transform_node_index, scroll_node_index, scroll_offset, tree);
-    // Run mutation callbacks to respond to updated scroll offset.
-    Mutate(CurrentBeginFrameArgs().frame_time);
-  }
+      property_trees->element_id_to_scroll_node_index[element_id];
+  property_trees->scroll_tree.OnScrollOffsetAnimated(
+      layer_id, transform_node_index, scroll_node_index, scroll_offset, tree);
+  // Run mutation callbacks to respond to updated scroll offset.
+  Mutate(CurrentBeginFrameArgs().frame_time);
 }
 
 bool LayerTreeHostImpl::AnimationsPreserveAxisAlignment(