cc: Change scoped_ptr<T>() to nullptr.

cc/test/layer_tree_pixel_test.cc

Index: cc/test/layer_tree_pixel_test.cc
diff --git a/cc/test/layer_tree_pixel_test.cc b/cc/test/layer_tree_pixel_test.cc
index ed14e33f0e031fe3649f8176083503a1ff46907f..2f293f7858857978c092be1cb84082972ef67219 100644
--- a/cc/test/layer_tree_pixel_test.cc
+++ b/cc/test/layer_tree_pixel_test.cc
@@ -130,8 +130,7 @@ void LayerTreePixelTest::EndTest() {
   // Drop TextureMailboxes on the main thread so that they can be cleaned up and
   // the pending callbacks will fire.
   for (size_t i = 0; i < texture_layers_.size(); ++i) {
-    texture_layers_[i]->SetTextureMailbox(TextureMailbox(),
-                                          scoped_ptr<SingleReleaseCallback>());
+    texture_layers_[i]->SetTextureMailbox(TextureMailbox(), nullptr);
   }
 
   TryEndTest();