cc: Move LayerImpl::AddChild and RemoveChild to LayerImplTestProperties

cc/trees/occlusion_tracker_unittest.cc

Index: cc/trees/occlusion_tracker_unittest.cc
diff --git a/cc/trees/occlusion_tracker_unittest.cc b/cc/trees/occlusion_tracker_unittest.cc
index e371e54c0e344194531bd4218e385bde8baf940c..37b1d96281ad8ff15d3be14f3488a1014733ccc0 100644
--- a/cc/trees/occlusion_tracker_unittest.cc
+++ b/cc/trees/occlusion_tracker_unittest.cc
@@ -122,7 +122,7 @@ class OcclusionTrackerTest : public testing::Test {
     std::unique_ptr<LayerImpl> layer = LayerImpl::Create(tree, id);
     LayerImpl* layer_ptr = layer.get();
     SetProperties(layer_ptr, transform, position, bounds);
-    parent->AddChild(std::move(layer));
+    parent->test_properties()->AddChild(std::move(layer));
     return layer_ptr;
   }
 
@@ -157,7 +157,7 @@ class OcclusionTrackerTest : public testing::Test {
         layer_ptr->SetOpaqueContentsRect(gfx::Rect());
     }
 
-    parent->AddChild(std::move(layer));
+    parent->test_properties()->AddChild(std::move(layer));
     return layer_ptr;
   }