Remove scroll layer ids from scrollbar layers

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index da95603c411cd0f2f4c7d71393d1bfdff67f6756..8bd315fddeb2a38d93308134874c363f76146f97 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -1731,7 +1731,9 @@ void LayerTreeImpl::RegisterScrollbar(ScrollbarLayerImplBase* scrollbar_layer) {
         scroll_element_id);
   }
 
-  DidUpdateScrollState(scrollbar_layer->ScrollLayerId());
+  // TODO(pdr): Refactor DidUpdateScrollState to use ElementIds instead of
+  // layer ids and remove this use of LayerIdByElementId.
+  DidUpdateScrollState(LayerIdByElementId(scroll_element_id));
 }
 
 void LayerTreeImpl::UnregisterScrollbar(