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

cc/layers/surface_layer.cc

Index: cc/layers/surface_layer.cc
diff --git a/cc/layers/surface_layer.cc b/cc/layers/surface_layer.cc
index 385f158608fd06b6565a5dcf32de4281a464aa32..f345b7423ddbe58e0665bd8d5740a343ca9ef08e 100644
--- a/cc/layers/surface_layer.cc
+++ b/cc/layers/surface_layer.cc
@@ -12,11 +12,12 @@ 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();
 }
@@ -26,7 +27,7 @@ scoped_ptr<LayerImpl> SurfaceLayer::CreateLayerImpl(LayerTreeImpl* tree_impl) {
 }
 
 bool SurfaceLayer::DrawsContent() const {
-  return surface_id_ && Layer::DrawsContent();
+  return !surface_id_.is_null() && Layer::DrawsContent();
 }
 
 void SurfaceLayer::PushPropertiesTo(LayerImpl* layer) {