Replace LayerImpl::sorting_context_id with transform reference.

cc/test/layer_tree_json_parser_unittest.cc

Index: cc/test/layer_tree_json_parser_unittest.cc
diff --git a/cc/test/layer_tree_json_parser_unittest.cc b/cc/test/layer_tree_json_parser_unittest.cc
index e600eef95bbe00a7d4a7f9670161d5bc70ac6dcf..9b0477a035ca7e6c9adfd403d2665aab2ae8c146 100644
--- a/cc/test/layer_tree_json_parser_unittest.cc
+++ b/cc/test/layer_tree_json_parser_unittest.cc
@@ -84,6 +84,7 @@ TEST_F(LayerTreeJsonParserSanityCheck, Basic) {
   parent->test_properties()->AddChild(std::move(child));
   root_impl->test_properties()->AddChild(std::move(parent));
   tree->SetRootLayerForTesting(std::move(root_impl));
+  tree->BuildPropertyTreesForTesting();
 
   std::string json = host_impl.LayerTreeAsJson();
   scoped_refptr<Layer> root = ParseTreeFromJson(json, NULL);
@@ -111,6 +112,7 @@ TEST_F(LayerTreeJsonParserSanityCheck, EventHandlerRegions) {
 
   root_impl->test_properties()->AddChild(std::move(touch_layer));
   tree->SetRootLayerForTesting(std::move(root_impl));
+  tree->BuildPropertyTreesForTesting();
 
   std::string json = host_impl.LayerTreeAsJson();
   scoped_refptr<Layer> root = ParseTreeFromJson(json, NULL);