Update cc effect node documentation.

cc/layers/render_surface_impl.cc

Index: cc/layers/render_surface_impl.cc
diff --git a/cc/layers/render_surface_impl.cc b/cc/layers/render_surface_impl.cc
index eba8543beccce7560c21c79ecb772ad3d5282bcd..b02397f31f24f469a9c4adbd13014b5d5e94dfeb 100644
--- a/cc/layers/render_surface_impl.cc
+++ b/cc/layers/render_surface_impl.cc
@@ -127,6 +127,8 @@ LayerImpl* RenderSurfaceImpl::MaskLayer() {
 }
 
 bool RenderSurfaceImpl::HasMask() const {
+  // TODO(wkorman): Shouldn't we use Layer::INVALID_ID rather than
+  // kInvalidNodeId below?
   return OwningEffectNode()->mask_layer_id != EffectTree::kInvalidNodeId;
 }