Workaround transform node NPE in draw_property_utils::UpdateElasticOverscroll

cc/trees/draw_property_utils.cc

Index: cc/trees/draw_property_utils.cc
diff --git a/cc/trees/draw_property_utils.cc b/cc/trees/draw_property_utils.cc
index 15535c53448ed3030494b59e025a7dccaada694f..43120259e3db8674761c76254eedd46c3418a0a0 100644
--- a/cc/trees/draw_property_utils.cc
+++ b/cc/trees/draw_property_utils.cc
@@ -479,6 +479,13 @@ static void UpdateElasticOverscrollInternal(
 
   TransformNode* node = property_trees->transform_tree.Node(
       overscroll_elasticity_layer->transform_tree_index());
+  DCHECK(node);
+
+  // TODO(pdr): This is a workaround for https://crbug.com/721772 to avoid
+  // crashing when there's no transform node. This workaround should be removed.
+  if (!node)
+    return;
+
   if (node->scroll_offset == gfx::ScrollOffset(elastic_overscroll))
     return;