cc: Move LayerImpl::AddChild and RemoveChild to LayerImplTestProperties

cc/input/scrollbar_animation_controller_thinning_unittest.cc

Index: cc/input/scrollbar_animation_controller_thinning_unittest.cc
diff --git a/cc/input/scrollbar_animation_controller_thinning_unittest.cc b/cc/input/scrollbar_animation_controller_thinning_unittest.cc
index 7e5a0e446b17e5a18ec176ab5cdac4c91d98d787..4b77f6d7f0fe00cf9bbc61fce1a6b0bb60e28306 100644
--- a/cc/input/scrollbar_animation_controller_thinning_unittest.cc
+++ b/cc/input/scrollbar_animation_controller_thinning_unittest.cc
@@ -61,8 +61,8 @@ class ScrollbarAnimationControllerThinningTest
             kTrackStart, kIsLeftSideVerticalScrollbar, kIsOverlayScrollbar);
     scrollbar_layer_ = scrollbar.get();
 
-    scroll_layer->AddChild(std::move(scrollbar));
-    clip_layer_->AddChild(std::move(scroll_layer));
+    scroll_layer->test_properties()->AddChild(std::move(scrollbar));
+    clip_layer_->test_properties()->AddChild(std::move(scroll_layer));
     host_impl_.active_tree()->SetRootLayer(std::move(clip));
 
     scrollbar_layer_->SetScrollLayerId(scroll_layer_ptr->id());