Update cc effect node documentation.

cc/layers/render_surface_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layers/render_surface_impl.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 109 matching lines @@
   return DebugColors::SurfaceBorderWidth(
       layer_tree_impl_ ? layer_tree_impl_->device_scale_factor() : 1);
 }
 
 LayerImpl* RenderSurfaceImpl::MaskLayer() {
   int mask_layer_id = OwningEffectNode()->mask_layer_id;
   return layer_tree_impl_->LayerById(mask_layer_id);
 }
 
 bool RenderSurfaceImpl::HasMask() const {
+  // TODO(wkorman): Shouldn't we use Layer::INVALID_ID rather than
+  // kInvalidNodeId below?
   return OwningEffectNode()->mask_layer_id != EffectTree::kInvalidNodeId;
 }
 
 const FilterOperations& RenderSurfaceImpl::Filters() const {
   return OwningEffectNode()->filters;
 }
 
 gfx::PointF RenderSurfaceImpl::FiltersOrigin() const {
   return OwningEffectNode()->filters_origin;
 }
@@ skipping 398 matching lines @@
         NOTIMPLEMENTED();
         break;
       default:
         NOTREACHED();
         break;
     }
   }
 }
 
 }  // namespace cc