Use a struct for cc::Surface ids for more type safety

cc/layers/surface_layer.cc

 // Copyright 2014 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/surface_layer.h"
 
 #include "cc/layers/surface_layer_impl.h"
 
 namespace cc {
 
 scoped_refptr<SurfaceLayer> SurfaceLayer::Create() {
   return make_scoped_refptr(new SurfaceLayer);
 }
 
-SurfaceLayer::SurfaceLayer() : Layer(), surface_id_(0) {}
+SurfaceLayer::SurfaceLayer() : Layer() {
+}
 
 SurfaceLayer::~SurfaceLayer() {}
 
-void SurfaceLayer::SetSurfaceId(int surface_id) {
+void SurfaceLayer::SetSurfaceId(SurfaceId surface_id) {
   surface_id_ = surface_id;
   SetNeedsPushProperties();
 }
 
 scoped_ptr<LayerImpl> SurfaceLayer::CreateLayerImpl(LayerTreeImpl* tree_impl) {
   return SurfaceLayerImpl::Create(tree_impl, id()).PassAs<LayerImpl>();
 }
 
 bool SurfaceLayer::DrawsContent() const {
-  return surface_id_ && Layer::DrawsContent();
+  return !surface_id_.is_null() && Layer::DrawsContent();
 }
 
 void SurfaceLayer::PushPropertiesTo(LayerImpl* layer) {
   Layer::PushPropertiesTo(layer);
   SurfaceLayerImpl* layer_impl = static_cast<SurfaceLayerImpl*>(layer);
 
   layer_impl->SetSurfaceId(surface_id_);
 }
 
 }  // namespace cc