Replace LayerImpl::sorting_context_id with transform reference.

cc/layers/layer_impl_test_properties.cc

 // Copyright 2016 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/layers/layer_impl_test_properties.h"
 
 #include "cc/layers/layer_impl.h"
 #include "cc/output/copy_output_request.h"
 #include "cc/trees/layer_tree_impl.h"
 
 namespace cc {
 
 LayerImplTestProperties::LayerImplTestProperties(LayerImpl* owning_layer)
     : owning_layer(owning_layer),
       double_sided(true),
       force_render_surface(false),
       is_container_for_fixed_position_layers(false),
       should_flatten_transform(true),
       hide_layer_and_subtree(false),
       opacity_can_animate(false),
       num_descendants_that_draw_content(0),
+      sorting_context_id(0),
       num_unclipped_descendants(0),
       opacity(1.f),
       blend_mode(SkBlendMode::kSrcOver),
       scroll_parent(nullptr),
       clip_parent(nullptr),
       mask_layer(nullptr),
       parent(nullptr) {}
 
 LayerImplTestProperties::~LayerImplTestProperties() {}
 
@@ skipping 18 matching lines @@
 
 void LayerImplTestProperties::SetMaskLayer(std::unique_ptr<LayerImpl> mask) {
   if (mask_layer)
     owning_layer->layer_tree_impl()->RemoveLayer(mask_layer->id());
   mask_layer = mask.get();
   if (mask)
     owning_layer->layer_tree_impl()->AddLayer(std::move(mask));
 }
 
 }  // namespace cc