Replace LayerImpl::sorting_context_id with transform reference.

cc/layers/layer_impl_unittest.cc

Index: cc/layers/layer_impl_unittest.cc
diff --git a/cc/layers/layer_impl_unittest.cc b/cc/layers/layer_impl_unittest.cc
index 174f2e2ce4f65bbd90c81bed4e0859d8cb7323d8..a353a2032dadc1dd951aa423838e8e463596d051 100644
--- a/cc/layers/layer_impl_unittest.cc
+++ b/cc/layers/layer_impl_unittest.cc
@@ -233,7 +233,6 @@ TEST(LayerImplTest, VerifyLayerChangesAreTrackedProperly) {
   EXECUTE_AND_VERIFY_SUBTREE_DID_NOT_CHANGE(root->SetMasksToBounds(true));
   EXECUTE_AND_VERIFY_SUBTREE_DID_NOT_CHANGE(
       root->SetPosition(arbitrary_point_f));
-  EXECUTE_AND_VERIFY_SUBTREE_DID_NOT_CHANGE(root->Set3dSortingContextId(1));
   EXECUTE_AND_VERIFY_SUBTREE_DID_NOT_CHANGE(root->SetContentsOpaque(true));
   EXECUTE_AND_VERIFY_SUBTREE_DID_NOT_CHANGE(root->SetDrawsContent(true));
   EXECUTE_AND_VERIFY_SUBTREE_DID_NOT_CHANGE(root->SetBounds(bounds_size));
@@ -329,8 +328,6 @@ TEST(LayerImplTest, VerifyNeedsUpdateDrawProperties) {
                                       layer->NoteLayerPropertyChanged());
   VERIFY_NEEDS_UPDATE_DRAW_PROPERTIES(layer2->SetPosition(arbitrary_point_f);
                                       layer->NoteLayerPropertyChanged());
-  VERIFY_NEEDS_UPDATE_DRAW_PROPERTIES(layer->Set3dSortingContextId(1);
-                                      layer->NoteLayerPropertyChanged());
   VERIFY_NEEDS_UPDATE_DRAW_PROPERTIES(
       layer->SetBackgroundColor(arbitrary_color));
   VERIFY_NEEDS_UPDATE_DRAW_PROPERTIES(
@@ -357,7 +354,6 @@ TEST(LayerImplTest, VerifyNeedsUpdateDrawProperties) {
   VERIFY_NO_NEEDS_UPDATE_DRAW_PROPERTIES(layer->SetContentsOpaque(true));
   VERIFY_NO_NEEDS_UPDATE_DRAW_PROPERTIES(
       layer2->SetPosition(arbitrary_point_f));
-  VERIFY_NO_NEEDS_UPDATE_DRAW_PROPERTIES(layer->Set3dSortingContextId(1));
   VERIFY_NO_NEEDS_UPDATE_DRAW_PROPERTIES(layer->SetDrawsContent(true));
   VERIFY_NO_NEEDS_UPDATE_DRAW_PROPERTIES(
       layer->SetBackgroundColor(arbitrary_color));