Workaround transform node NPE in draw_property_utils::UpdateElasticOverscroll

cc/trees/draw_property_utils.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/draw_property_utils.h"
 
 #include <stddef.h>
 
 #include <vector>
 
@@ skipping 461 matching lines @@
     PropertyTrees* property_trees,
     const LayerType* overscroll_elasticity_layer,
     const gfx::Vector2dF& elastic_overscroll) {
   if (!overscroll_elasticity_layer) {
     DCHECK(elastic_overscroll.IsZero());
     return;
   }
 
   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;
 
   node->scroll_offset = gfx::ScrollOffset(elastic_overscroll);
   node->needs_local_transform_update = true;
   property_trees->transform_tree.set_needs_update(true);
 }
 
 #if DCHECK_IS_ON()
 static void ValidatePageScaleLayer(const Layer* page_scale_layer) {
@@ skipping 571 matching lines @@
 void UpdateElasticOverscroll(PropertyTrees* property_trees,
                              const Layer* overscroll_elasticity_layer,
                              const gfx::Vector2dF& elastic_overscroll) {
   UpdateElasticOverscrollInternal(property_trees, overscroll_elasticity_layer,
                                   elastic_overscroll);
 }
 
 }  // namespace draw_property_utils
 
 }  // namespace cc